I take it someone has already corrected your boo-boo about Harry Holland. Helen Clark was referring to arch-conservative PM Sid Holland, and not the Marxist Labour leader who died in 1933. [Can we fix this? Yes we can! - C.D. Sludge.]
Also, you do tend to repeat the fallacy that, before the campaign, Labour polled 56%. She really only polled 56% of about 70%, which means, at the peak of her pre-campaign support, only 39 of 100 persons polled said they would vote Labour.
The campaign did not so much see Labour's support erode. Rather it saw Labour and National receiving very few votes from those who had been undecided. This firming of the undecideds in favour of the smaller parties is the norm; it was always easily predicted.
The problem lies in the mainstream media's refusal to even mention the proportion of pollees who are undecided.