diff --git a/conda/dev-environment.yml b/conda/dev-environment.yml
index 15b97f0..2e3eb2f 100644
--- a/conda/dev-environment.yml
+++ b/conda/dev-environment.yml
@@ -13,3 +13,4 @@ dependencies:
     - mule
     - cfunits
     - dask
+    - f90nml
diff --git a/conda/meta.yaml b/conda/meta.yaml
index 965ba53..b7281a9 100644
--- a/conda/meta.yaml
+++ b/conda/meta.yaml
@@ -23,6 +23,7 @@ requirements:
         - scipy
         - sparse
         - xarray
+        - f90nml
 
 test:
     requires:
diff --git a/setup.py b/setup.py
index 99c9fc6..d0aa895 100644
--- a/setup.py
+++ b/setup.py
@@ -23,5 +23,6 @@
             ],
         entry_points = {
             'console_scripts': [
+                'diffnml = coecms.diffnml:main'
                 ]}
         )
diff --git a/src/coecms/diffnml.py b/src/coecms/diffnml.py
new file mode 100644
index 0000000..bd61c1a
--- /dev/null
+++ b/src/coecms/diffnml.py
@@ -0,0 +1,74 @@
+#!/usr/bin/env python
+# Copyright 2019 ARC Centre of Excellence for Climate Extremes
+# author: Scott Wales <scott.wales@unimelb.edu.au>
+#
+# 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 f90nml
+import argparse
+from collections import OrderedDict
+
+
+def diffnml(a, b):
+    """
+    Create a diff of two f90nml instances
+
+    Returns an ordered dict mapping tuples
+
+       (section, key) -> (left, right)
+
+    for (section, key) pairs where there is a difference, with left and right
+    being the values of that (section, key) from namelists a and b respectively
+
+    If that (section, key) is missing from one file then that file will report
+    None
+    """
+    groups = set([*a.keys(), *b.keys()])
+    output = OrderedDict()
+
+    for g in groups:
+        a_g = a.get(g, {})
+        b_g = b.get(g, {})
+
+        keys = set([*a_g.keys(), *b_g.keys()])
+        for k in keys:
+            a_k = a_g.get(k, None)
+            b_k = b_g.get(k, None)
+
+            if a_k != b_k:
+                output[(g,k)] = (a_k, b_k)
+
+    return output
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser(description="Creates a diff of two namelist files")
+    parser.add_argument('filea', metavar='FILE1')
+    parser.add_argument('fileb', metavar='FILE2')
+    args = parser.parse_args(argv)
+
+    print("< %s"%args.filea)
+    print("> %s"%args.fileb)
+
+    a = f90nml.read(args.filea)
+    b = f90nml.read(args.fileb)
+
+    for section, diff in diffnml(a,b).items():
+        group, key = section
+        left, right = diff
+        print(f'&{group}%{key}:\n\t<   {left}\n\t>   {right}')
+
+
+if __name__ == '__main__':
+    main()
+
diff --git a/test/test_diffnml.py b/test/test_diffnml.py
new file mode 100644
index 0000000..1018b27
--- /dev/null
+++ b/test/test_diffnml.py
@@ -0,0 +1,69 @@
+#!/usr/bin/env python
+# Copyright 2019 ARC Centre of Excellence for Climate Extremes
+# author: Scott Wales <scott.wales@unimelb.edu.au>
+#
+# 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 __future__ import print_function
+
+from coecms.diffnml import *
+import f90nml
+import io
+
+def make_nml(string):
+    return f90nml.read(io.StringIO(string))
+
+
+def test_diffnml():
+    """
+    Check diff logic
+    """
+    a = make_nml("""
+    &foo
+    bar = 1
+    /
+    """)
+    b = make_nml("""
+    &foo
+    bar = 'a'
+    baz = 1
+    /
+    """)
+
+    d = diffnml(a,b)
+
+    assert d[('foo','bar')] == (1, 'a')
+    assert d[('foo','baz')] == (None, 1)
+
+
+def test_main(tmpdir, capsys):
+    """
+    Check output formatting
+    """
+    a = tmpdir.join('a')
+    b = tmpdir.join('b')
+
+    a.write("""
+    &foo
+    bar = 1
+    /
+    """)
+    b.write("""
+    &foo
+    bar = 2
+    /
+    """)
+
+    main([str(a),str(b)])
+    
+    captured = capsys.readouterr()
+    assert captured.out == f"< {str(a)}\n> {str(b)}\n&foo%bar:\n\t<   1\n\t>   2\n"