A nationwide test of the Emergency Mobile Alert hit mobile phones across New Zealand tonight.
The alert landed just after 6pm and is part of a plan to ensure the Emergency Mobile Alert system works efficiently.
A number of tests have been sent out since 2017 - with some more successful than others.
The first alert was sent in November 2017 and reached just 34 per cent of cellphones in New Zealand.
Then in November 2018 another alert was sent - repeatedly in some cases - in error to Vodafone customers at 1.30am.
A second test that day reached about 60 per cent of cellphones - or about 3.6 million phones - received the warning which was a dramatic improvement from the first test.
At the time Civil Defence Minister Kris Faafoi said international experience showed once the alerts hit 70 per cent penetration a "critical mass" was achieved "in which you generate word of mouth that quickly spreads to just about everyone".
Tonight's test alert was sent by the Ministry of Civil Defence and Emergency Management.
About four million phones were expected to make "a loud noise" marking the annual national test.
Earlier, MCDEM director Sarah Stuart-Black said last year six in 10 New Zealanders received the alert, and that number was expected to climb further today.
She said those people whose mobile phones were capable of receiving the alerts could expect to hear a loud, penetrating sound, and a notification would display.
The test alert linked to the Civil Defence website, where people can fill out a brief survey about how they experienced the alert.
Survey results will inform future improvements to the system and gather feedback for mobile phone providers.
"Fast and reliable information is crucial when emergencies strike. Emergency Mobile Alert is a vital information channel for alerting people to threats to their life, health or property," said Stuart-Black.
"The nationwide test is a way to test our systems, the cell towers and your phone's ability to receive an Emergency Mobile Alert.
"Some people will be a little more used to the system, as this is now our third nationwide test, and the system has also been used a number of times for local emergencies, such as the Auckland Convention Centre fire.
"For others, it will be the first time they've experienced it."
Last month the system was used in Auckland during the fire at the New Zealand International Convention Centre which brought the CBD to a standstill.
An alert was sent out by Fire and Emergency New Zealand during the incident - highlighting a feature of the system that can ringfence an area to receive the message and define the time it is active.
The Emergency Mobile Alert uses cell broadcast technology, which is also used in a number of countries including the United States, Japan, Chile and the Netherlands.
Stuart-Black said there was no need to subscribe or download an app.
"All you need is a mobile phone that is capable of receiving Emergency Mobile Alerts, and a network signal," she said.
"It uses internationally proven technology that isn't susceptible to overloading, which is vital when we need to rapidly alert a large number of people.
"However, no form of technology is completely failsafe, so it doesn't replace other alerting channels such as radio or social media, or the need to act upon natural warning signs. Remember – if an earthquake is long or strong, get gone.
However, not everyone will receive Emergency Mobile Alerts.
"If you're in a mobile coverage zone and don't receive an alert, that doesn't mean there's something wrong with your phone or the system," Stuart-Black explained.
"It may be because your phone isn't compatible or doesn't have the latest updates.
"For phones bought overseas or parallel imported, your alert may display differently – for example, some may show up as "Presidential Alert".
"In local emergencies, only those in the geographically targeted area will receive it.
As Emergency Mobile Alert is about keeping people safe, no one can opt out.
"Your phones may show optional settings used in other countries, but in New Zealand we
will use a special broadcast channel that is permanently on," said Stuart-Black.
If you feel your life is in danger, don't wait for an official warning.