Knock-off Mongrel Mob gang patches are being sold on online, and an expert is warning that people would be "foolish" to wear them in public.

A seller on Chinese-based website Aliexpress has the iron-on, embroidered patches on sale for $50 each.

Purchase history shows customers in New Zealand, Australia and Ireland have bought the patches, which read 'Mongrel Mob Aotearoa' alongside its unmistakable bulldog symbol.

Harry Tam, who works with gangs across New Zealand, told Newshub he "hates to think" what the implications could be if someone wore one.

"Common sense would say don't do it," he says.

"Obviously this has never happened before but I'm sure if people do it, they'll no doubt find out in due course what the consequences are. When it comes down to it, the people purchasing these are putting themselves in jeopardy."

He says people would be "bloody foolish" to wear the patches.

The seller, BTC Co Limited, is based in Guangdong, China. The company's website describes itself as a garments manufacturer established in 1997, specialising in men's and boys' apparel.

But Mr Tam says a patch is much more than a fashion statement.

"For them to have a patch that doesn't belong to them is beyond stupid. There's no mana behind the patch they have. They're not entitled to it, it's the intellectual property right of the group."

Every gang has their own rules to earning a patch, but most have a "probationary period", Mr Tam says.

"It's really checking their commitment to the crew."

The patches don't have one particular supplier - members in different parts of New Zealand wear leather, print or embroidery, and for the most part they're made locally.

Mr Tam says the Mongrel Mob isn't the only gang targeted and other patches are being sold as well.

"All I can say is, get rid of them, return to sender, and ask for your money back."

The seller has been contacted by Newshub but has not responded.

Newshub.