Iain, I think it could be worthy of having a close look at the badges worn by the NZ Tunnellers.
Interestingly, the motto of the NZ Engineers "Quo fas et gloria ducunt" (Whither right and glory lead) is inscibed on the outter circle of the cap badge. So it seems logical that "Inga Wahi Katoa" (Everywhere/Ubique) was used as officer collar badges.
I have only one photo of a tunneller officer, but cannot make out the letters on the collar badge, so wondering if your tunneller officer photos are any better?
I will include a pre embarkation roll of NZ Tunnelling officers just in case other forum members might have photos.
Just out of interest, when the NZ Tunnelling Company was first formed at Avondale (Auckland) they were known as the "Red Feds" as amoung its volunteers were no less than 11 former secratries of various trade unions, and around 40 members who were members of trade union committees. Most of the men were miners who learned their business without Osh looking over their shoulders, so yes they were a hard bunch of ba$tards.
Evening Post, 21 February 1917
December 1915, List of officers.
Major John Evelyn Duigan, New Zealand Staff Corps.
Capt. Lachlan Bain Campbell.
Capt. Daniel Black Waters.
Capt. Hugh Vickerman.
Lieut. Robert Henry Daldy.
Lieut. Norman Thompson.
Lieut. John Dudley Holmes.
Lieut. Arthur James Wigley.
Lieut. Henry Ernest Metcalfe.
Lieut. Harold Watkinson.
Lieut. Robert Henry Price Ronayne.
Lieut. William Mellish Durant
Lieut. Guy Shackburgh Collyns.
Lieut. Leslie May.
Lieut Gregory Whittaker King.
Lieut. James Campbell Neill.
Major (temporary) Colin Huntly Gordon, New Zealand Medical Corps.