diff --git a/packages/data/src/pyearthtools/data/load.py b/packages/data/src/pyearthtools/data/load.py
index 42f572ef..f29c1aab 100644
--- a/packages/data/src/pyearthtools/data/load.py
+++ b/packages/data/src/pyearthtools/data/load.py
@@ -42,11 +42,22 @@ def load(stream: Union[str, Path], **kwargs) -> "pyearthtools.data.Index":
         (pyearthtools.data.Index):
             Loaded Index
     """
+    
+    # Check if the stream is not a Path or String and raise an error
+    if not isinstance(stream, (str, Path)):
+        raise TypeError(
+            f"Stream is not a Path or String {type(stream)} - {stream}."
+        )
+
     contents = None
 
+    # Check is the stream is a path
     if os.path.sep in str(stream):
         try:
+            # Check if the path is a directory
             if parse_path(stream).is_dir():
+                
+                # Create a list of config files if found. 
                 stream = list(
                     [
                         *Path(stream).glob("catalog.cat"),
@@ -54,10 +65,9 @@ def load(stream: Union[str, Path], **kwargs) -> "pyearthtools.data.Index":
                         *Path(stream).glob("*.cat"),
                         *Path(stream).glob("*.edi"),
                     ]
-                )[
-                    0
-                ]  # Find default save file of index
-
+                )[0]  # Find default save file of index
+            
+            # Combine all files into a single variable
             contents = "".join(open(str(parse_path(stream))).readlines())
         except FileNotFoundError as e:
             raise e
@@ -66,6 +76,7 @@ def load(stream: Union[str, Path], **kwargs) -> "pyearthtools.data.Index":
         except IndexError:
             raise FileNotFoundError(f"No default catalog could be found at {stream!r}.")
 
+    # If the stream is not a path, check if it is a string and store it in contents. 
     if contents is None:
         contents = str(stream)
 
@@ -74,4 +85,5 @@ def load(stream: Union[str, Path], **kwargs) -> "pyearthtools.data.Index":
 
     contents = initialisation.update_contents(contents, **kwargs)
 
-    return yaml.load(str(contents), initialisation.Loader)
+    # Load and return the contents as yaml.
+    return yaml.load(str(contents), initialisation.Loader)
\ No newline at end of file
diff --git a/packages/data/tests/test_load.py b/packages/data/tests/test_load.py
new file mode 100644
index 00000000..ad2658b0
--- /dev/null
+++ b/packages/data/tests/test_load.py
@@ -0,0 +1,100 @@
+# 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.
+
+import pytest
+from unittest.mock import patch, mock_open
+
+import yaml
+
+from pyearthtools.data import load 
+from pyearthtools.data.utils import parse_path
+from pathlib import Path
+
+
+
+def test_load_invalid_stream_type():
+    """Test loading with an invalid type for the stream."""
+
+    with pytest.raises(TypeError):
+        load(1234) 
+
+def test_load_file_not_found():
+    """Test loading a file from a directory that does not exist."""
+    
+    mock_file_path = "data/nonexistent_file.yaml"
+    with patch("pyearthtools.data.utils.parse_path", return_value=Path(mock_file_path)):            
+         with pytest.raises(FileNotFoundError):
+                load(mock_file_path)
+
+def test_load_empty_directory():
+    """Test loading from a directory with no matching catalog files."""
+
+    mock_empty_dir = "data/empty_dir"
+
+    with patch("pyearthtools.data.utils.parse_path", return_value=Path(mock_empty_dir)), \
+         patch("pathlib.Path.glob", return_value=[]):   # Mock an empty directory
+            with pytest.raises(FileNotFoundError):
+                load(mock_empty_dir)
+
+
+def test_update_contents_called_correctly():
+    """Test that initialisation.update_contents is called with the correct arguments."""
+    mock_contents = "key: value"
+    mock_updated_contents = "updated key: value"
+
+    # Mock the `initialisation.update_contents` function
+    with patch("pyearthtools.utils.initialisation.update_contents", return_value=mock_updated_contents) as mock_update_contents:
+        # Mock the `yaml.load` function to avoid parsing errors
+        with patch("yaml.load", return_value={"key": "value"}):
+            # Call the `load` function with a string stream
+            result = load(mock_contents, extra_arg="test")
+
+            # Assert that `initialisation.update_contents` was called with the correct arguments
+            mock_update_contents.assert_called_once_with(mock_contents, extra_arg="test")
+
+            # Assert that the result of `load` matches the mocked updated contents
+            assert result == {"key": "value"}
+
+
+def test_load():
+    """Test load function"""
+
+    mock_file_content = "key: value"
+
+    # Mock dependencies
+    mock_open_function = mock_open(read_data=mock_file_content)
+    mock_parse_path = Path("valid_file.yaml")
+    mock_yaml_load = {mock_file_content}
+
+    with patch("builtins.open", mock_open_function), \
+         patch("os.path.sep", "/"), \
+         patch("pyearthtools.data.utils.parse_path", return_value=mock_parse_path), \
+         patch("yaml.load", return_value=mock_yaml_load):
+
+        # Call the load.py load function with mocked dependencies.
+        result = load("valid_file.yaml")
+
+    # Assert the result
+    assert result == {mock_file_content}
+
+
+def test_load_invalid_yaml():
+    """Test loading invalid YAML content."""
+
+    invalid_yaml_content = "invalid: yaml: content"
+    with patch("pyearthtools.utils.initialisation.update_contents", return_value=invalid_yaml_content), \
+         patch("yaml.load", side_effect=yaml.YAMLError):
+            with pytest.raises(yaml.YAMLError):
+                load(invalid_yaml_content)
+