Thousands of gun owners will be affected by a decision to reclassify many weapons as military-style semi-automatics.

The guns involved were previously defined as "sporting configuration" and owners did not need to notify police of the model of each gun they owned, as is required of owners of military-style semi-automatics.

The reclassification focuses on guns that have the function of a free-standing pistol grip - even though the pistol grip may be attached to a stock - enabling the firearm to be used as an assault weapon.

Police superintendent Tony McLeod said he did not know how many owners and guns were affected, but expected it to run into the thousands.

New Zealand has more than 200,000 licensed firearm owners but there is no way of knowing how many guns they have because of a decision in 1982 to abandon a system of licensing each gun in favour of licensing gun owners.

Owners of military-style semi-automatics undergo more stringent vetting than for standard A-class endorsed licences, and are required to have a more expensive and secure safe for their weapons.

Mr McLeod said the reclassification was not an attempt to create a firearms registration system by default, but better tracking of these types of weapons may be a result.

Crown Law is understood to have been consulted during the reclassification process, part of which came into effect on June 9.

Those buying these guns now need to have the tougher E-class endorsed licence, obtain a permit to buy and advise police of the make and model.

Police had yet to decide how best to compel existing owners of these guns to comply, though there was likely to be a period of grace before owners faced the possibility of being fined.

Mr McLeod said it was "a definite risk" that some owners may not come forward because of the higher cost of the E-class licence.

The owner of internet gun auction site GunStuff, Mark Fogarin, said the reclassification was "causing much confusion and frustration throughout New Zealand's firearms community".

The change could cost some owners about $1000 for upgrading their licences and safety equipment.

Mr Fogarin said the change targeted licensed gun owners rather than criminals and would have no effect on people such as Jan Molenaar, who shot dead police officer Len Snee and injured three other men in Napier last month.

Molenaar's gun licence had lapsed, he was unfit to hold a licence and yet had an arsenal of 18 guns including military-style semi-automatics.

Police have traced three of the weapons to burglaries of licensed gun owners' premises.

Mr Fogarin said the flaw in that tragedy was that no check was made after Molenaar's licence expired to ensure he no longer had guns.

Mr McLeod said cases such as that of Coromandel Peninsula collector John Mabey added to the debate, but were not the reason for the change.

Mabey reported 132 guns stolen but later admitted selling 30 restricted firearms to unlicensed people and was sentenced to 21 months' jail. He is defending charges of having made a false burglary complaint.

Several weapons from his collection have turned up in criminal hands. One was used to shoot at police and another was believed to have been used in a fatal shooting.