Civil Defence will test its emergency mobile phone alert system this evening.

Between 6pm and 7pm, mobile phones that are capable of receiving the alert will emit a loud sound and a message will appear on screen.

The sound of the test alert will be distinct and noisy, but should not cause too much fright or distraction to anyone driving or otherwise preoccupied.

Four million phones across the country are expected to pick up the test alert for emergencies.

People do not need to sign up to receive the message - the system uses broadcast technology to send alerts to mobile phones in specific locations.

Civil Defence and Emergency Management director Sarah Stuart-Black said the annual tests were an opportunity to increase coverage of the broadcast and make sure the system worked.

Only six out of 10 phones picked up the alert last year.

However, she said the 2018 percentage rate was considered a success and exceeded expectations, given that it was only the second year the technology had been used in New Zealand.