Winston Peters has issued a warning to gang members who say they won't be handing over their guns despite the Government's crackdown.

The Deputy Prime Minister said on Monday that official legislation had been agreed upon by Cabinet to ban military-style semi-automatic weapons and assault rifles, among other gun law reforms.

Prime Minister Jacinda Ardern announced the gun law reforms over a week ago after it was revealed the alleged Christchurch gunman had legally purchased assault rifles using a standard A Category gun licence.

The Arms (Prohibited Firearms, Magazines and Parts) Amendment Bill has marked September 30 as the date an amnesty period will end for owners to hand their weapons to police, and a Government buy-back scheme will be announced.

The buy-back scheme will take into account the age and type of weapon, and the market value. It is estimated it will cost between $100 million and $200 million.

The challenge for the Government will be getting everyone to co-operate - particularly gangs - and that looks to be unlikely, with a Mongrel Mob leader telling Stuff members will not be complying with the law.

"Will gangs get rid of their weapons? No. Because of who we are, we can't guarantee our own safety," Sonny Fatu, president of the Waikato branch of the Mongrel Mob, said.

"The attacks between our organisations are gang-on-gang, they do not involve the non-gang members," Fatu said, adding that members of the estimated 1000 associates should be trusted not to harm innocent people.