Hundreds of people wanting to witness the third and final reading of the legislation legalising same sex marriage are expected at Parliament on Wednesday night.
Parliament's sergeant-at-arms Brent Smith said there had been plenty of interest from members of the public wanting to secure seats.
He said while the 205 seats above the main Debating Chamber could usually be booked, the rules had changed for Wednesday to first in, first served.
Parliament had also arranged for an overflow room, which would hold 200 people and show the parliamentary debate on the big screen.
Watching from a separate room will be 150 guests of the MP who introduced the bill, Louisa Wall.
Mr Smith said posters and banners will not make it through security.
If passed, the bill will have a four-month commencement period.
Conrad Reyners of the Campaign for Marriage Equality, expects gay people to start getting married four months from now when the bill becomes law.
Mr Reyners says he does acknowledge the law change is opposed by many people.
"Some of those beliefs are genuinely held. But what I would say is if you actually just take a bit of time, in four months time the sun will still rise and the birds will still sing and I think what you'll find is that the only consequence out of this bill is that loving committed couples will be able to enjoy the instutition of marriage, just like every other New Zealander and be afforded the same equal rights."
Mr Reyners says his campaign will keep rallying right up to the last minute.
MPs will have a conscience vote on the issue, and if they stick with their previous positions, the bill will pass by a clear majority.