Police in Nelson are cracking down on the use of motorised bicycles, some of which have been breaking the law.
They said the bikes were becoming a trend, which was a concern because a crash involving one could lead to serious injuries or death, as happened near Motueka in 2014.
Nelson Bays senior sergeant Scott Richardson said two disqualified drivers in Nelson had recently been arrested and charged after they were stopped while riding motorised bicycles.
One of them had been travelling at more than 60km/h in a 50km/h zone and police in Canterbury had noticed dozens of the bikes appearing, Mr Richardson said.
Two people stopped in Nelson in recent weeks were a sign the problem was growing locally.
Mr Richardson said the bikes, which were usually home-adapted and included an engine, presented a couple of major hazards, including that it was difficult for motorists to judge their speed.
"They're not a very safe vehicle. If they crash they don't respond like a motorcycle's designed to - and that can cause very serious injuries.
"Most of the people riding them aren't wearing motorcycle helmets - they're wearing bicycle helmets which just aren't designed for the speeds these vehicles can cruise along at.
"The kinetic energy when you're pushing a bike along at 50km/h can cause very serious injuries if it hits another cyclist or a pedestrian," he said.
Mr Richardson said a coroner's report into the death of Thomas Patrick McCarthy near Motueka would be released soon.
Mr McCarthy died after crashing his homemade motorised bicycle into a ditch on Brooklyn Valley Road near Motueka in 2014. He had been wearing a helmet, Mr Richardson said.
"They aren't bicycles - they're effectively motorbikes, and wherever a motorcycle shouldn't be allowed, these shouldn't be either," Mr Richardson said.
The motorised bikes were not a loophole for disqualified drivers, he said.
"There's no way it's a legal vehicle for a disqualified driver."