Funny thing is that no-one was actually expecting Muldoon to win 1975. Labour's majority in 1972 was considered simply too large (note that the reverse happened in 1978 - no-one was expecting Labour to get close).
Possible options:
(1) No Muldoon in 1975 (Muldoon's campaign was perhaps the most effective in New Zealand history). Perhaps have Marshall lose less badly in 1972, and keep enough support in caucus to get another go in 1975.
(2) Kirk lives. Kirk was far more charismatic than Rowling, and would avoid the "weak little intellectual" attacks launched on his successor. Kirk's sudden illness wasn't expected either, to the point where there remain conspiracy theories about CIA involvement.
(3) Kirk doesn't cancel the 1973 Springbok Tour. Kirk in 1972 had promised not to interfere in the tour - only to change his mind a year later. Muldoon played on this endlessly in 1975. Flipside is that you get the nastiness of apartheid South Africa turning up, with international repercussions (African boycott of the 1974 Commonwealth Games) and problems on the streets. I think short of the NZRFU calling off the Tour on its own accord, this is a bit of a no-win situation for Labour. Perhaps Kirk doesn't make his 1972 promise, wins anyway, and invokes the fact that the Nats cancelled 1967?
(4) Limit the external economic shocks. Lessen the oil crisis, keep Britain out of the EEC, and you really help Labour (Muldoon was considered an economic wizard in 1975, so was able to play on the sense of crisis).
(5) Rowling's Government actually promotes the Compulsory Savings Scheme effectively, and irons out the difficulties in advance. Nip the Dancing Cossacks in the bud.