A parliamentary select committee is hearing oral submissions on the government's fast-tracked firearm law changes today.
The Finance and Expenditure Select Committee will consider possible changes to the bill and report it back to Parliament on Monday.
Deputy Police Commissioner Mike Clement has already explained to the committee how the Christchurch mosque accused was able to buy his guns and magazines legally with his category A gun licence, and then bring them together to create an illegal combination.
He said the government's bill would make it a lot harder to convert one type of firearm into another.
He also said it would close a loophole in current law that allowed gun importers and owners make minor modifications to their firearms to prevent them from being categorised as military, and therefore restricted.
In the first of the hearings today, Fish and Game Council raised concerns the government was not leaving enough time to process all guns to be handed in as a result of the firearms buy-back scheme.
Fish and Game's chief executive Martin Taylor told the committee he supported the ban and government buy-back, but raised concern about the six-month deadline.
He said if it took 30 minutes to process one firearm, and 30,000 firearms were handed back, the task may be too great.
The committee's chairperson, Labour MP Michael Wood, said the committee would hear 15 oral submissions today and consider all the written submissions. The deadline for written submissions closes tomorrow.
He said the vast majority of the 250,000 or so licensed firearm owners would not be impacted by the new legislation, as it specifically dealt with military-style assault weapons.
Police Association president Chris Cahill said he's unhappy about the exemptions in the government's proposed gun law change and said firearms collectors pose the biggest risk.
He said collectors only had to remove a part from a firearm and make it inoperable but this isn't safe, as it's possible to reactivate them.
He showed a photo of 74 Uzis he said were imported by a collector with the idea that they would be used as props.
He said they were all operable firearms and were only stopped at the border because the exporter from overseas alerted customs to the guns, and to previous consignments.
Gun City owner David Tipple, who sold the accused man four firearms, told the select committee that banning many semi-automatic firearms would mean the killer would win.
"Rushing this good feel law is causing division, it is bad law and it will result in serious injustices - worse it ignores what went wrong, there are no loop holes in existing laws. This happened because he broke numerous laws."
Gun rights blogger Mike Loder railed against the reform, telling MPs the Prime Minister is a "tyrant" and calling gun safety campaigners "scumbags".
Mr Loder told MPs the proposed law change is rushed and ill-thought through, warning it could drive firearms underground.
He said the debate has been dishonest and emotive.
"I watched the Prime Minister literally laugh when she announced that new gun laws would be rushed through. A journalist said 'will people be able to make submissions', she literally laughed and went 'ha ha, hurry. That is a tyrant.'"
Questioned by the committee, he stood by earlier comments that some lobby groups supporting gun control were scum and that the prime minister is a tyrant.
Mr Wood pushed back, suggesting Mr Loder is a conspiracy theorist and pointing to an inflammatory blog post where he had linked gun control with Nazi Germany.
Yesterday gun owners expressed dismay the committee was allocating just one day to make oral submissions on the Arms (Prohibited Firearms, Magazines and Parts) Amendment Bill.
The committee requested oral submissions from "a representative group", but firearms owners and industry figures said the group would only represent a small number of people, given the short timeframe, which was unfair.
The government wants to pass the bill into law by Thursday next week.