A new blacklisting system will help stop the theft of mobile phones by reducing the street value of stolen devices, say police.

Vodafone, Telecom and 2degrees spent the last year working on a system which lets each mobile company block the unique I.D of a phone if it is reported stolen.

It means that, if blacklisted, a phone won't be able to be used on any of the three networks in New Zealand regardless if the SIM card in the back of a device is changed.

National manager of mobility for the police, Superintendent Steve Christian, said blacklisting system was a "great leap forward."

"There has, until now, been a significant gap in this area. We are pleased the telcos are now joining together to render stolen devices as being of no value on the streets," Christian said.

The New Zealand Telecommunications Forum (TCF), an industry group that encourages co-operation in the sector, launched the system today.

TCF chief executive David Stone said people who want a lost or stolen phone on the blacklist should contact their mobile company.

Stone warned people should be careful if they weren't purchasing phones from a registered dealer:

"If you purchase a stolen mobile phone or other device - even if you think you are buying it legitimately - you may not have any recourse if that device is subsequently blocked, and so you could end up losing money.

"We urge people to purchase mobiles only from registered dealers or from sources they know they can trust."