I don't think hes just infantry. Body armour seems typical for those who have to clean up mines etc.
I doubt it was for mine clearing. I suspect it was to give further protection against shrapnel and bomb splinters.
Edit:
An Australian chaplain wearing the "Large Box Respirator" also known as the "Respiratory Tower" during the First World War. The man is probably Major Walter Dexter, DSO, DCM, in the Bois Grenier Sector on 5th June 1916.