I always use Penrite oil. Even became an agent for them, with many repeat orders and excellent feedback. It's an Australian company, so it should be easy to get their stuff in NZ. For a 1947 P2, you need Penrite GB 40, specified where SAE40 oils were originally recommended and equivalent to a 90 weight gearbox oil. It's marketed over here at £7.95 per litre, in 1l. and 5l. packs. Perhaps cheaper down under?
For pre-1940 'boxes,use Penrite Transoil 90.
That said, the previous owner of my first Rover, a '37 10 acquired back in 1981, recommended cheap mineral engine oil for all P1and P2 gearboxes, provided the seals are ok. I took his word for it and used it for the next 10 years with no problems. He also reckoned that the same cheap stuff was best in the Luvax jar as it was less likely to lead to clogging.
1939 20 H.P. Salmons Tickford D.H.C.