Royal New Zealand Service Corps.
This appears to be gilt anodised aluminium. Whilst supposedly a boon to the soldier in that it needed no polishing to stay bright, the finish did not wear well.
The New Zealand Army Service Corps was formed in 1910 to provide logistic support to the army. It received the Royal prefix in 1946. In 1979 it was disbanded, the duties now undertaken by the Royal New Zealand Army Logistic Regiment.
Post & Telegraph Corps and Department
I haven’t found a record that P & T Corps had its own button, although they did have a badge depicting a winged foot. Therefore, the button probably belong to the P & T Department.
For a good summary of this 1911-1921 corps, see https://nzhistory.govt.nz/media/photo/post-and-telegraph-corps-badge and https://nzhistory.govt.nz/war/first-world-war-postal-service
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