Four weeks to the day since 51 people were gunned down at two Christchurch mosques, gun law reforms come into force.

It follows the fastest act of law-making in our country on a major piece of legislation in recent history.

In the space of two weeks, the Government saw its bill through three readings, a 48-hour window for thousands of written submissions, and one day of oral submissions.

A Havard professor says he can't imagine a day U.S. gun laws change with the kind of speed we've seen here.

Director of Harvard injury research centre David Hemenway says New Zealand's shown it can react immediately to a mass shooting, unlike the U.S.

"It makes a lot of sense that now is the time to do something, so it's very remarkable, at least from the US standpoint, that New Zealand can do something so quickly."

He says any potential future law change there would never happen with the same kind of haste as here.

"There'd be so much pushback. One of our major parties is very much aligned with the gun lobby, so it's very hard to get much done."

Under the amnesty, people have more than five months to hand their now illegal firearms in.

If they were legally acquired, the owners will be compensated under a gun buy-back scheme.

But what's not known is how many banned guns and parts are out there, and how much the buy-back scheme will cost the Government.