Lake Illawarra licensing Sergeant Gary Keevers said police recently seized the black machine gun, similar in style to an M4 military rifle, after it was sent by courier to a gun dealer in Unanderra.

A Cringila man bought the gun from a Queensland seller through an online gun marketplace, and had it sent to the Unanderra dealer by a courier registered to deliver firearms.

Sgt Keevers said the gun was not able to be registered or owned in NSW, as it featured an illegal telescoping stock and a laser sight with a power exceeding the allowable limit.

The gun was also what Sgt Keevers called a "military-style firearm", which cannot be possessed by regular gun owners.

"The gun was sent to the dealer in Unanderra, who contacted police as he has a lawful obligation to notify police of the arrival of a prohibited firearm," he said.

"The dealer hasn't done anything wrong, but he alerted police and we seized the gun."

Sgt Keevers said police were investigating whether the Cringila purchaser knew the gun was illegal in NSW, but said a check with police could help other gun owners avoid similar situations.

"Before you go onto any website or purchase a gun, make inquiries with licensing police or the firearms registry to make sure it is legal," he said.

"Our message is, don't buy until you do the checks."