Dear Ms. Cornelissen,
Thank you for posting your request on History Hub!
We suggest that you contact the British National Archives (UK), National Archives of Australia, and the New South Wales (NSW) State Archives to see if they can offer you any assistance with your search.
Additionally, FamilySearch offers a number of resources related to researching British Military Records that you might find useful.
We hope this information is helpful, and best of luck with your family research!