diff --git a/packages/pipeline/src/pyearthtools/pipeline/operations/dask/split.py b/packages/pipeline/src/pyearthtools/pipeline/operations/dask/split.py
index 4069c10c..25742625 100644
--- a/packages/pipeline/src/pyearthtools/pipeline/operations/dask/split.py
+++ b/packages/pipeline/src/pyearthtools/pipeline/operations/dask/split.py
@@ -106,9 +106,7 @@ def split(self, sample: da.Array) -> tuple[da.Array]:
     def join(self, sample: tuple[da.Array]) -> da.Array:
         """Join `sample` together"""
 
-        data = da.stack(sample, axis=0)
-        data = da.moveaxis(data, 0, self.axis)
-        return data
+        return da.concatenate(sample, axis=self.axis)
 
 
 # class VSplit(Spliter, DaskOperation):
diff --git a/packages/pipeline/src/pyearthtools/pipeline/operations/numpy/split.py b/packages/pipeline/src/pyearthtools/pipeline/operations/numpy/split.py
index 1fee2dd2..bab763bf 100644
--- a/packages/pipeline/src/pyearthtools/pipeline/operations/numpy/split.py
+++ b/packages/pipeline/src/pyearthtools/pipeline/operations/numpy/split.py
@@ -107,9 +107,7 @@ def split(self, sample: np.ndarray) -> tuple[np.ndarray]:
     def join(self, sample: tuple[np.ndarray]) -> np.ndarray:
         """Join `sample` together"""
 
-        data = np.stack(sample, axis=0)
-        data = np.moveaxis(data, 0, self.axis)
-        return data
+        return np.concat(sample, axis=self.axis)
 
 
 class VSplit(Spliter):
@@ -135,7 +133,8 @@ def __init__(
         self.record_initialisation()
 
     def split(self, sample: np.ndarray) -> tuple[np.ndarray]:
-        return np.vsplit(sample)  # type: ignore
+        # splits into equally-sized subsections.
+        return np.vsplit(sample, indices_or_sections=sample.shape[0])  # type: ignore
 
     def join(self, sample: tuple[np.ndarray]) -> np.ndarray:
         """Join `sample` together"""
@@ -165,7 +164,10 @@ def __init__(
         self.record_initialisation()
 
     def split(self, sample: np.ndarray) -> tuple[np.ndarray]:
-        return np.hsplit(sample)  # type: ignore
+        # splits into equally-sized subsections.
+
+        nsections = sample.shape[min(1, sample.ndim - 1)]  # np.hsplit will split along axis 0 if ndims==1
+        return np.hsplit(sample, indices_or_sections=nsections)  # type: ignore
 
     def join(self, sample: tuple[np.ndarray]) -> np.ndarray:
         """Join `sample` together"""
diff --git a/packages/pipeline/src/pyearthtools/pipeline/operations/xarray/split.py b/packages/pipeline/src/pyearthtools/pipeline/operations/xarray/split.py
index 5a240139..d152bd18 100644
--- a/packages/pipeline/src/pyearthtools/pipeline/operations/xarray/split.py
+++ b/packages/pipeline/src/pyearthtools/pipeline/operations/xarray/split.py
@@ -43,7 +43,7 @@ def __init__(
                 Kwargs needed for merge on the `undo`. Defaults to None.
         """
         super().__init__(
-            recognised_types=(xr.DataArray, xr.Dataset),
+            recognised_types=(xr.Dataset,),
             recursively_split_tuples=True,
         )
         self.record_initialisation()
@@ -90,5 +90,5 @@ def __init__(
     def split(self, sample: T) -> tuple[T, ...]:
         return tuple(sample.sel(**{self.coordinate: i}) for i in sample.coords[self.coordinate])
 
-    def undo(self, sample: tuple[T, ...]) -> xr.Dataset:
-        return xr.merge(sample, **(self._merge_kwargs or {}))
+    def join(self, sample: tuple[T, ...]) -> xr.Dataset:
+        return xr.concat(sample, dim=self.coordinate)
diff --git a/packages/pipeline/tests/operations/dask/test_dask_split.py b/packages/pipeline/tests/operations/dask/test_dask_split.py
new file mode 100644
index 00000000..59cf3d4f
--- /dev/null
+++ b/packages/pipeline/tests/operations/dask/test_dask_split.py
@@ -0,0 +1,49 @@
+# Copyright Commonwealth of Australia, Bureau of Meteorology 2025.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from pyearthtools.pipeline.operations.dask import split
+
+import dask.array as da
+import pytest
+
+
+@pytest.fixture(scope="module")
+def example_data():
+    return da.array(range(2 * 3 * 4)).reshape((2, 3, 4))
+
+
+def test_onaxis(example_data):
+    """Tests dask OnAxis split operation class."""
+    op = split.OnAxis(axis=1)
+
+    # try join before splitting
+    with pytest.raises(RuntimeError):
+        op.join((example_data, example_data))
+    result = op.split(example_data)
+    assert all((arr == example_data[:, d, :]).all().compute() for d, arr in enumerate(result))
+
+    orig = op.join(result)
+    assert (orig == example_data).all().compute()
+
+
+def test_onslice(example_data):
+    """Tests dask OnSlice split operation class."""
+    slices = ((0, 1), (1, 2), (2, 4))
+    op = split.OnSlice(*slices, axis=2)
+    result = op.split(example_data)
+    for sl, arr in zip(slices, result, strict=True):
+        assert (arr == example_data[:, :, sl[0] : sl[1]]).all().compute()
+
+    orig = op.join(result)
+    assert (orig == example_data).all().compute()
diff --git a/packages/pipeline/tests/operations/numpy/test_numpy_split.py b/packages/pipeline/tests/operations/numpy/test_numpy_split.py
new file mode 100644
index 00000000..88ee96b5
--- /dev/null
+++ b/packages/pipeline/tests/operations/numpy/test_numpy_split.py
@@ -0,0 +1,67 @@
+# Copyright Commonwealth of Australia, Bureau of Meteorology 2025.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from pyearthtools.pipeline.operations.numpy import split
+
+import numpy as np
+import pytest
+
+
+@pytest.fixture(scope="module")
+def example_data():
+    return np.array(range(2 * 3 * 4)).reshape((2, 3, 4))
+
+
+def test_onaxis(example_data):
+    """Tests numpy OnAxis split operation class."""
+    op = split.OnAxis(axis=1)
+
+    # try join before splitting
+    with pytest.raises(RuntimeError):
+        op.join((example_data, example_data))
+    result = op.split(example_data)
+    assert all(np.array_equal(arr, example_data[:, d, :]) for d, arr in enumerate(result))
+
+    orig = op.join(result)
+    assert np.array_equal(orig, example_data)
+
+
+def test_onslice(example_data):
+    """Tests numpy OnSlice split operation class."""
+    slices = ((0, 1), (1, 2), (2, 4))
+    op = split.OnSlice(*slices, axis=2)
+    result = op.split(example_data)
+    for sl, arr in zip(slices, result, strict=True):
+        assert np.array_equal(arr, example_data[:, :, sl[0] : sl[1]])
+
+    orig = op.join(result)
+    assert np.array_equal(orig, example_data)
+
+
+def test_vsplit(example_data):
+    """Tests numpy VSplit split operation class."""
+    op = split.VSplit()
+    result = op.split(example_data)
+    assert all(np.array_equal(arr, result[i]) for i, arr in enumerate(result))
+    orig = op.join(result)
+    assert np.array_equal(orig, example_data)
+
+
+def test_hsplit(example_data):
+    """Tests numpy HSplit split operation class."""
+    op = split.HSplit()
+    result = op.split(example_data)
+    assert all(np.array_equal(arr, example_data[:, i : i + 1, :]) for i, arr in enumerate(result))
+    orig = op.join(result)
+    assert np.array_equal(orig, example_data)
diff --git a/packages/pipeline/tests/operations/xarray/test_xarray_split.py b/packages/pipeline/tests/operations/xarray/test_xarray_split.py
new file mode 100644
index 00000000..d2cc063b
--- /dev/null
+++ b/packages/pipeline/tests/operations/xarray/test_xarray_split.py
@@ -0,0 +1,74 @@
+# Copyright Commonwealth of Australia, Bureau of Meteorology 2025.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from pyearthtools.pipeline.operations.xarray import split
+
+import numpy as np
+import xarray as xr
+import pytest
+
+
+@pytest.fixture(scope="module")
+def example_dataarray():
+    return xr.DataArray(
+        np.array(range(2 * 3 * 4)).reshape((2, 3, 4)), coords={"c0": range(2), "c1": range(3), "c2": range(4)}
+    )
+
+
+@pytest.fixture(scope="module")
+def example_dataset(example_dataarray):
+    return xr.Dataset({"a": example_dataarray, "b": 2 * example_dataarray})
+
+
+def test_onvariables(example_dataset):
+    """Tests xarray OnVariables operation class."""
+
+    # split on all variables
+    op = split.OnVariables()
+    result = op.split(example_dataset)
+    assert result[0].equals(example_dataset.drop_vars("b"))
+    assert result[1].equals(example_dataset.drop_vars("a"))
+
+    # join datasets
+    orig = op.join(result)
+    assert orig.equals(example_dataset)
+
+    # split on selected variables
+    op = split.OnVariables(variables=("a",))
+    result = op.split(example_dataset)
+    assert result[0].equals(example_dataset.drop_vars("b"))
+
+    # split on non-existent variable
+    op = split.OnVariables(variables=("c",))
+    with pytest.raises(ValueError):
+        op.split(example_dataset)
+
+
+def test_oncoordinate(example_dataarray, example_dataset):
+    """Tests xarray OnCoordinate operation class."""
+    op = split.OnCoordinate("c1")
+    result = op.split(example_dataset)
+    for i, arr in enumerate(result):
+        assert arr["a"].equals(example_dataset["a"].loc[:, i, :])
+        assert arr["b"].equals(example_dataset["b"].loc[:, i, :])
+
+    orig = op.join(result)
+    assert orig.broadcast_equals(example_dataset)
+
+    result = op.split(example_dataarray)
+    for i, arr in enumerate(result):
+        assert arr.equals(example_dataarray.loc[:, i, :])
+
+    orig = op.join(result)
+    assert orig.broadcast_equals(example_dataarray)