Members of Parliament will cast conscience votes on two high-profile social issues in two days - and Prime Minister John Key has warned his MPs it will get "very emotive".
Last night Key remained personally behind a split alcohol purchase age and in favour of gay marriage, but support for a change to the drinking age may be faltering.
He has warned fellow National Party MPs that debate on the "marriage equality" bill in particular is set to get "very emotionally charged" with "a lot of email correspondence".
"There will be a deluge of email campaigns both ways on this issue."
The first reading of a bill allowing gay marriage is expected tomorrow night and it appears to have enough support to pass.
Some MPs are understood to have privately indicated support, but are reluctant to publicly declare their backing for fear of attracting particularly strong lobbying.
However, the heat is set to go up on all MPs when the votes are cast and debate rages through the remaining stages of the bill. Emails already sent to MPs cite a "destructive homosexual political cult", the "gay mafia" and even suggest the bill is "about legalised child abuse".
"You do not know that there [is a] silent majority who do not support gay marriage. Human rights do not equal marriage rights," one email says.
Others in support of the bill cite equal opportunities regardless of sexuality.
"The vast majority of Kiwis . . . know it's time for full equality for gay and lesbian Kiwis and they want to finally see their friends enjoy the same rights as them," a supporter says.
Green MP Kevin Hague said few of those writing to MPs against gay marriage were presenting "strong arguments or arguments that you wouldn't expect".
Some amounted to "an expression of a view with a bit of menace attached", he said.
Hague, who has led political action for homosexual law reform in the past, said he was well used to seeing strong opinions on the matter. "But someone maybe seeing it for the first time . . . might find it pretty hard to deal with."
A vote on the alcohol purchase age, also a conscience issue for all but the Green Party MPs, is scheduled to follow the gay marriage bill with a debate on Thursday night. MPs will get to vote for the age to go up to 20; move to a split age of 18 at bars and pubs and 20 at bottle stores and supermarkets or to remain at 18 at all outlets.
MPs may get the unusual chance to vote on all three options simultaneously, with the least popular dropping out for a final run-off between the two remaining options.
Last night Key confirmed his support for a split age, which he admitted was "not perfect".
"There is an argument that says at 19, you'd be old enough to go off to the local bar and have a beer but if you decide to pick up a six-pack on the way to a BBQ you're not going to be legally allowed to do that," he said.
"I can understand the merits of why that seems a slightly odd position to be in but it's really about trying to say - look, we're trying to make a number of potential moves overall to try and reduce heavy binge drinking by very young people."