A High Court decision has set back efforts by police to impose greater controls on a wider range of firearms.
The court has ruled against a move by police to reclassify many "sporting configuration" firearms as military-style semi-automatics (MSSAs).
Police spent $60,000 advertising the changes. Owners would have had to
undergo more stringent vetting, register each affected weapon and have a more expensive gun safe.
But a High Court judgment released this week has ruled the police's reinterpretation does not comply with the Arms Act 1983.
The judge said existing law did not intend the wider application the police had introduced.
It was too early for police to comment on the possibility of an appeal.
"Police need to fully review the decision and only then will be in a position to determine the appropriate action," spokesman Jon Neilson said.
National Shooters Association president Richard Lincoln, who took the judicial review, said the police would have to scrap current plans to reinterpret what constitutes MSSAs.
His association wants the police hierarchy, who changed the definition, to repay the money spent on the advertising campaign.
The reclassification, part of which came into effect last June, focused on guns that have the function of a free-standing pistol grip even though the pistol grip may be attached to a stock.
The police said the move was in response to design changes that gave some sports guns similar functionality to MSSAs. That interpretation had been expected to affect thousands of gun owners.
"It seems to me, that in using the words it did, Parliament had in mind a particular kind of grip which it described in a confined way rather than the looser definition which the police now propose to apply," Justice Jill Mallon said.
"If manufacturers are now producing firearms intended to get around the restrictions, and if that is of concern to the police [and others], then it may be that the legislation needs amending."
Mr Lincoln had sought judicial clarification of classification of his Heckler & Koch SL8 rifle. The police took the view Mr Lincoln's gun was an MSSA.
