David Smitham wrote:
Firstly the NI main trunk railway was not completed until 1908, several years after this cover was mailed. The cover may have gone part way by train between Wellington and Auckland or gone all the way by ship.
David
David - you are correct that the Main Trunk was not complete at that date but I still believe the letter probably went by train. The Wellington Evening Post for 23rd July 1900 suggests that mail for Auckland could be carried by the Manawatu train on either the 24th or 26th. Whether it did or not we will never know.

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The mystery is partly solved however thanks to Mark Benvie who was quick to spot that the letter was probably sent by a Sgt. JDG Shera of the 8th Company, 4th NZ Contingent whilst somewhere in transit on a troopship to South Africa (Boer War). Sgt Shera's address on his Volunteers' Attestation form is given as "Remuera, Auckland" (as his father's address).
A check of the back of the envelope does indeed show the initials "JDGS" in the same hand as the address written on the front.
As a serving soldier letters with no stamps on would attract only the single 2½d rate (this was reduced to 1d later in 1900).
What is unusual however is that the letter (to attract the concessionary rate, rather than the usual 'double deficiency') should have been countersigned by a commanding officer. The only thing we can summise is that all the troops on board the troopship wrote a letter and rather than countersign everyone the commanding officer provided a covering letter for a sealed mailbag containing all the letters.
A little further checking on the web and I discovered that the Fourth Contingent left Wellington aboard SS "Gymeric" at the end of March.
The 4th were in South Africa from around the end of May and Shera was there through 1901 and recognised for "Gallantry in Attack" at Pongola Bosch, October 23rd 1901.
This does pose another problem as to how the letter would be dated on arrival in Wellington in July when the writer, by that date, had been in South Africa for almost two months. The only likely explanation is that the letter was written by Sargeant Shera in transit or on arrival in South Africa and the mail took some time to get back to New Zealand.
However a nice cover with, it turns out, a nice story attached. Thanks to Mark for spotting the crucial link.
chippin