New Zealand's gay marriage bill is on track to become law after passing its second reading with a strong majority.

MPs cast conscience votes and the count late on Wednesday night was 77-44.

On its first reading in August the count was 80-40.

Labour MP Louisa Wall, who drafted the Marriage (Definition of Marriage) Amendment Bill, says lobbying has been "ferocious" with MPs' inboxes clogged and some systems crashing.

She opened the second reading debate with an appeal for equality.