There are fears gun owners and retailers will find loopholes in the semi-automatic weapons ban.
Overseas gun makers have found ways around the law in the past, says president of the Police Association Chris Cahill.
"We know that when states in America have brought in restrictions on types of semi-automatics, or particular types of weapons, gun-makers have found ways to adapt, to get around these rules," he told Newshub on Sunday.
The Government is fast-tracking new legislation to outlaw the kinds of weapons used the Christchurch attacks in March, which left 50 people dead. A buyback scheme will be set up later this year so present owners can be compensated.
NZME on Sunday reported there are several devices available in the US which can make lethal killing machines technically legal, without sacrificing their potential to slaughter.
One is the 'Kali Key', which turns a semi-automatic rifle into a bolt-action rifle that can still fire rapidly, and can easily be converted back.
Some gun manufacturers have even reportedly invented new firing mechanisms to get around restrictions.
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Cahill says the wording in the new legislation will need to be clear on what is allowed and what isn't, and it'll need to cover future potential modifications.
"It's on the onus of the gun owner to prove his or her weapon is legal, rather than the police."
MPs from both sides of the House have pushed back at criticism the legislation is being rushed.
ACT's David Seymour was planning to hold up Parliament's rush to change the law, saying New Zealand was being turned into a "de facto dictatorship". He failed however, when he spent too long telling reporters about his plans and missed the vote.
Labour MP Greg O'Connor, a former president of the Police Association, told Magic Talk on Sunday the debate's already been had - a select committee in 2017 made a number of recommendations which will become law next week.
"The select committee inquiry heard some very good evidence and came out with some very good recommendations, most of which Paula Bennett - who was the minister at the time - chose not to do, anything but four of them. It was a bit of a shame. So we've had all the debate."
National MP Lawrence Yule said he rejected accusations from some that this is somehow undemocratic".
"This has been very well-studied in the past… Every time - Labour, NZ First, National - every time we try to do this, it has got too politically hard due to pressure largely from the gun lobby. I think the time is up. We don't need military-style semi-automatic weapons in New Zealand."
O'Connor said in 2017 he "predicted that an overseas group would come here, see how slack our gun laws were, and come and commit an atrocity like this".
"It gives me no pleasure to be right. But what it means is we now have to act… We could only have done this, sadly, in the wake of this disaster. If we'd been passing these laws, proposing these laws right now… it would have been like the capital gains tax. We would have massive kickback from all the Opposition parties."
More changes will come later this year, including a buyback scheme expected to cost hundreds of millions of dollars.
Gun retailers aren't expected to be in line for compensation however, angering shop owners.
"A number of the dealers… are dealing with the commercial reality," said Yule. "They have a lot of stock on order. So far, Stuart Nash has said they're unlikely to be compensated for that. That's one of the things we're going to have to work out over the next few months."
Both O'Connor and Yule said they were in favour of a comprehensive register of gun owners.