New Zealand World War 1 Cover 1918 on Postal Stationery

This is an odd, peculiar piece of very old Great Britain postal stationery, mailed during World War I from New Zealand to a soldier in the New Zealand Expeditionery Force (N.Z.E.F.) notwithstanding its origination in New Zealand and its use of a cover imprinted with British postage. I don't understand why British postage was used to mail something from New Zealand to this soldier, but it was.

Imprinted with a One Penny red postage imprint with strong color. I'm not sure exactly what year this is from because I can't read the cancellation date, but 6/1/18 is handwritten just to the left of the center. It's addressed to a 2nd Lieutenant in the New Zealand Expeditionery Force.

There's a vertical tear where it was folded (was it put into the soldier's pocket folded up?) and other flaws -- see scans.

$5.50 postpaid to a US address, or $7.00 postpaid to anywhere else.