Civil Defence is tonight testing their emergency mobile alert system.

Between 6 and 7pm, around half of cell phones will automatically receive an alert.

Civil Defence director Sarah Stuart-Black says most people with a modern phone will hear it.

"What we know is, compared to last year, which was about 34 per cent about phones which received the message, this year we're expecting it to be about 50 per cent."

The alert uses different technology to a normal text message, and can be geographically targeted in the case of a local emergency.

She says that in the case of a real emergency, several groups have the ability to use the alert system.

"The Ministry of Civil Defence and Emergency Management, the Civil Defence Emergency Management groups around the country, so if they have something quite localised. Also Foreign Emergency New Zealand, New Zealand Police, Ministry of Health and Ministry for Primary Industries."

Stuart-Black says that in future, more people will receive the alert as they upgrade their mobile phones.