diff --git a/tests/test_output_sanitiser.py b/tests/test_output_sanitiser.py
index 7c7075c9..3fe8d892 100644
--- a/tests/test_output_sanitiser.py
+++ b/tests/test_output_sanitiser.py
@@ -1,11 +1,6 @@
-import os
-
-import genet.utils.io
 from genet import Network
 from genet.output import sanitiser
 from genet.output import spatial as spatial_output
-from geopandas import GeoDataFrame
-from shapely.geometry import Point
 
 
 def test_sanitising_list():
@@ -56,27 +51,3 @@ def test_sanitising_geodataframes_with_ids_list(assert_semantically_equal):
     assert_semantically_equal(
         links[["length", "from", "to", "id", "ids", "u", "v", "modes"]].to_dict(), correct_links
     )
-
-
-def test_saving_geodataframe_with_missing_geometry_produces_file(tmpdir):
-    expected_file_name = "tmp"
-    expected_output_path = tmpdir / expected_file_name + ".parquet"
-    assert not os.path.exists(expected_output_path)
-
-    data = {"id": ["1", "2"], "geometry": [float("nan"), Point(2, 1)]}
-    gdf = GeoDataFrame(data, crs="EPSG:4326")
-    genet.utils.io.save_geodataframe(gdf, filename=expected_file_name, output_dir=tmpdir)
-
-    assert os.path.exists(expected_output_path)
-
-
-def test_saving_geodataframe_with_missing_data_in_string_column_produces_file(tmpdir):
-    expected_file_name = "tmp"
-    expected_output_path = tmpdir / expected_file_name + ".parquet"
-    assert not os.path.exists(expected_output_path)
-
-    data = {"id": ["1", float("nan")], "geometry": [Point(2, 1), Point(2, 1)]}
-    gdf = GeoDataFrame(data, crs="EPSG:4326")
-    genet.utils.io.save_geodataframe(gdf, filename=expected_file_name, output_dir=tmpdir)
-
-    assert os.path.exists(expected_output_path)
diff --git a/tests/test_utils_io.py b/tests/test_utils_io.py
new file mode 100644
index 00000000..29623d1b
--- /dev/null
+++ b/tests/test_utils_io.py
@@ -0,0 +1,37 @@
+import os
+
+import genet.utils.io
+import pytest
+from geopandas import GeoDataFrame
+from shapely import Point
+
+
+def test_throws_error_when_filetype_is_not_supported():
+    with pytest.raises(RuntimeError) as e:
+        genet.utils.io.check_file_type_is_supported("hello")
+
+    assert "is not a supported file type" in str(e.value)
+
+
+def test_saving_geodataframe_with_missing_geometry_produces_file(tmpdir):
+    expected_file_name = "tmp"
+    expected_output_path = tmpdir / expected_file_name + ".parquet"
+    assert not os.path.exists(expected_output_path)
+
+    data = {"id": ["1", "2"], "geometry": [float("nan"), Point(2, 1)]}
+    gdf = GeoDataFrame(data, crs="EPSG:4326")
+    genet.utils.io.save_geodataframe(gdf, filename=expected_file_name, output_dir=tmpdir)
+
+    assert os.path.exists(expected_output_path)
+
+
+def test_saving_geodataframe_with_missing_data_in_string_column_produces_file(tmpdir):
+    expected_file_name = "tmp"
+    expected_output_path = tmpdir / expected_file_name + ".parquet"
+    assert not os.path.exists(expected_output_path)
+
+    data = {"id": ["1", float("nan")], "geometry": [Point(2, 1), Point(2, 1)]}
+    gdf = GeoDataFrame(data, crs="EPSG:4326")
+    genet.utils.io.save_geodataframe(gdf, filename=expected_file_name, output_dir=tmpdir)
+
+    assert os.path.exists(expected_output_path)