Civil Defence has sent out its first national emergency test to about two million New Zealand mobile phones tonight.

The "unique and penetrating sound" was intended to alert mobile phone users to the first national test of the new emergency civil defence alert system.

It was originally due to happen in April 2018 under a contract with the technology's developers in the Netherlands.

The alert test was brought forward by six months after the public was confused by conflicting information on the risk of a tsunami around New Zealand after the Kaikoura earthquake last November.

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Civil Defence and Emergency Management Ministry spokesman Anthony Frith said the Government had spent $18 million on the new mobile-based alert system, including an $800,000 publicity campaign to raise awareness of the first official test between 6pm and 7pm tonight.

Only a third of New Zealand's 5.8 million mobile phones are believed to be capable of receiving the alert - substantially fewer than originally planned.

But that number is expected to grow quickly as new mobile phones are sold and existing phones are upgraded.

The alert directed people to a text message which explained that it was a test, but which in a real emergency will warn people to leave for higher ground because of a tsunami, or to evacuate an area because of fire risk or a police emergency.

Test emergency alerts will appear on about two million NZ mobile phones tonight. Photo / Ministry of Civil Defence and Emergency Managament
Test emergency alerts will appear on about two million NZ mobile phones tonight. Photo / Ministry of Civil Defence and Emergency Managament

Frith said the technology had already been implemented in the Netherlands and in other earthquake-prone countries including Chile, Japan, the Philippines and the United States.

Many Vodafone subscribers in New Zealand received the alert by mistake on October 4 when the developers in the Netherlands sent a message out "in error" between 1am and 2am NZ time.

In contrast, today's test was planned between 6pm and 7pm to coincide with the main television news shows to maximise public awareness and minimise panic.

The alerts use a separate channel from normal text messages and will come through on most late-model phones, provided that their software has been updated recently.

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People can check whether their phone is set up for the alerts.

On an iPhone, open Settings, select Notifications, and at the bottom you should see a toggle for "Emergency Alerts".

On Android phones the settings vary between different phones but can be found under either Settings or Messages.

Although the phones indicate that the alerts can be turned off, Frith said the New Zealand system had actually been set up so that people could not opt out of it.