List of PMs
Same as Australia, except:
Frank Forde and Ben Chifley replaced with Peter Fraser
Malcolm Fraser is totally replaced by Robert Muldoon
Tony Abbott and Malcolm Turnbull replaced with John Key
(who will resign in late 2018 after being party leader for 10 years)
Next I will look into elections and see if NZ being part of Australia would have changed any federal election results (spoiler: the answer is yes, I know even not having looked yet, if it is somehow no I'll be shocked)
Also FTR, NZ's Parliament would have served as it's State Legislature.
It would have been reduced to 60 members after joining Australia, and would have had 79 members in 1993. It would have transitioned to AV along with the rest of Australian states, but, would have switched to Proportional Representation as it did IRL; but with 100 not 120 MPs. It also would have Maori seats as it does now.