Police are cracking down on Uber, the cheap and trendy new-kid-on-the-taxi rank, leaving paying customers on the pavement.

After complaints from the old-school taxi firms, police have begun fining the Uber drivers whose lower fares have been hurting the big cab companies.

The private car hire service has hit back, lodging a complaint of police harassment with the Independent Police Complaints Authority. A spokeswoman said police officers put passengers at risk by booting them out of the hired cars.

Uber has taken the world by storm, and is gaining a big chunk of market share in Auckland and Wellington. It is expected to launch in Christchurch and Queenstown this year. But in the biggest city, police confirm they have stopped several Uber drivers and charged them or issued them infringement notices for using their smartphone app as a meter - a breach that would make them subject to tough taxi regulations.

NZ Taxi Federation director Roger Heale said the police crackdown proved Uber drivers are operating illegally. "We know they've been warned. They can't claim this is out of the blue."

Uber operates as a private hire service which means the fare has to be set at the time of booking, rather than using a meter. This means Uber does not have to abide by taxi regulations, thus saving on operating costs.

Offending drivers could be fined $2000. "This could potentially cost the drivers their livelihood," Heale said. Uber has drawn both criticism and praise since launching in New Zealand at the start of 2014.

Uber spokeswoman Katie Curran said a number of precedents around the world concurred that smartphones were not taxi meters. "Uber believes policy makers in New Zealand could reasonably come to the same conclusion, rather than having this debate played out over a potentially lengthy period which is not in the public interest."

One particular police officer had targeted several drivers: "This officer's actions have also put riders at risk when he ordered them to vacate the vehicles, leaving them stranded at night with no other travel options to get home," Curran said.

Inspector Jim Wilson, Auckland City Police Acting District Commander, said police took public safety seriously. Passengers in affected vehicles were either dropped in central Auckland or driven home by police.

NZ Transport Agency national manager operations Kate Styles said they had spoken with Uber to ensure it was clear on its obligations.

Launched in the US in 2009, Uber has spread quickly but faced a backlash wherever it goes. The service was ordered to stop operating in Spain last month following accusations of unfair competition by local taxi drivers. In the same month, Uber was blacklisted in the Indian capital Delhi after a driver allegedly raped a passenger.