Speaker thieves mount sirens on their bikes to emulate the older men featured in these car-mounted speaker "battles". Police say they doubt these speakers are stolen due to the sophistication of the set-ups.

School and train station sirens have become a hot ticket item in an emerging black market, leaving communities in danger.

Scores of loudspeakers and fire alarms have also been stolen from community centres and petrol stations around central Auckland to be used in "battles".

The mobile sound systems are strapped to handlebars of bikes and used in competitions where the one with the loudest music is crowned "Siren King".

JONATHAN MILNE/FAIRFAX NZ The loudspeakers battles consist of two opponents each picking a song and playing them side by side through their sirens.

A Facebook page with nearly 3000 members has been set up to trade and sell the heavy-duty speakers. There have been 1127 sale posts since the page was created a year ago.

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Among the sale posts are discussions about where the sirens have been stolen from. "Who would buy two 15W [sirens] if I stole some…" posted one user.

FAIRFAX NZ A teenage craze has escalated into a fully-fledged subculture with its own black market and competitions.

Another member claimed to have a siren and fire alarm from a train station which they're "keen to swap for a phone or offers". Other posts include orders for specific sirens which are usually accompanied with photos of wads of cash.

An industry insider says 90 per cent of the sirens listed on the Facebook page look stolen.

"Some of them are quite expensive pieces of equipment. There are speakers on there that have complicated wiring and are the kind of thing you would usually see on top of a lamp post at the corner of a rugby field."

The source says there has been a huge influx of siren sales throughout Auckland over the last 12 months.

"Most are after a specific siren that is compact and can be easily fixed to a bike or vehicle."

The sirens can easily be modified to run off a cell phone or through an amplifier hooked up to a car battery.

Inspector Steve Samuels says the police have received about 70 calls concerning excessive noise coming from speakers attached to bikes. He says police can confiscate the speakers when they look like they've been stolen and keep them until the owner can prove they legally purchased them.

However many youths are aware of their tactics.

"We had a boy who purchased a speaker legitimately but then photocopied the receipt for all his mates. They are keeping us on our toes at all times."