People around the world have purchased the knock-off Mongrel Mob t-shirts from AliExpress.

Knock-off Mongrel Mob t-shirts are being sold online, but an academic warns that wearing one could be "be extremely detrimental to one's health".

Different iterations of cotton t-shirts and hooded jumpers bearing the gang's insignia are available on AliExpress and Wish.com for between $20 to $50.

But Jarrod Gilbert, a University of Canterbury senior lecturer and author of Patched: The history of gangs in New Zealand, said wearing one could "involve the the removal of that clothing from you and probably some violence to go with it".

SUPPLIED Police urged shoppers to consider the potential risk of being perceived as associating with a gang.

Gilbert said a gang's patch was incredibly highly prized.

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"They fight to earn them and they fight to retain them," he said.

"They'll also fight anyone who is seen to be wearing them when they haven't, in their eyes, earned them. So it would be extremely detrimental to one's health to wear a Mongrel Mob t-shirt that you buy off the web."

AliExpress' site said the garments were made in China and had been sold to dozens of buyers in New Zealand, the Netherlands and Canada.

Flags, caps and patches for the Mongrel Mob and other gangs such as the Headhunters and Bandidos were also available.

IAIN McGREGOR/STUFF Jarrod Gilbert said a "street form of justice" could be handed out if anyone who was not a gang member was seen wearing a patch.

Gilbert said the knock-offs would concern the gangs and while many New Zealand gangs didn't have trademarks on their insignia, "there's a street form of justice to be paid if you breach copyright".

New Zealand police said it was not illegal to wear, buy or sell gang insignia but police "would remind those who choose to wear it of their obligations under the relevant legislation and bylaws.

"We would also urge them to consider the potential risks of being perceived as associating with a gang," a police spokeswoman said.

In 2009, the Whanganui District Council introduced a bylaw barring people wearing gang insignia in public places.

However, the High Court ruled in 2011 the ban was unlawful and invalid as it had effectively removed gang members from the district and failed to fully take into account people's rights around freedom of expression.