Auckland Police are on high alert after suspicious vehicle activity around Helensville

Reports of dishonesty offences - and theft of vehicles and possessions in them, have been increasing in recent weeks.

Senior Sergeant Roger Small says just before two this morning, a car failed to stop when signalled on State Highway 16.

He says it was spiked and all the occupants arrested.

Small says yesterday, also about 2am, a vehicle was spotted speeding through roadworks near Helensville.

He's warning locals to lock vehicles - and not store valuables in them.