Gun rights advocates are claiming around two-thirds of banned firearms are still in circulation.
The buyback inspired by the March terror attack in Christchurch finishes this week, after five months of collections.
About 47,000 weapons have been handed in, but the Council of Licenced Firearms Owners says that falls well short.
"Seventy percent of 170,000 firearms are going to quite possibly enter the grey market, and if they enter the grey market, that will feed the black market," spokesperson Nicole McKee told Newshub.
She said the buyback policy has been rushed, and is based on ideological remedies rather than reality.
"It's really important that some of these firearms - well, all of these firearms - that are prohibited get handed in. But it's highly unlikely that's going to occur in the next five days."
She says the scheme is looking like a failure.
"We just think there are so many people out there that don't realise they have those prohibited firearms, and that in itself is a bigger issue."
It's not actually known exactly how many guns are in Kiwis' hands, as until now owners haven't had to register each individual firearm.
The deadline is this Friday. Police Minister Stuart Nash has said there will be no leniency for those who fail to hand in their illegal weapons.
Newshub has contacted Police Minister Stuart Nash for comment.