It comes down to one person pushing a button - before a lot of people get a big surprise.
The decision to issue mobile alerts after events such as Friday's earthquakes is reserved for trained operators in a few agencies. And if people don't get one when they think they should have - they're not shy at letting people know.
A Civil Defence alert message on Friday morning came to Gisborne residents three hours after a severe 7.1-magnitude earthquake shook them awake.
The 5.20am alert said they no longer needed to evacuate for a tsunami threat, after an earthquake struck at 2.27am off the North Island's east coast, 105km east of Te Araroa.
That was the first alert message Gisborne residents received. Residents between Cape Runaway and Tolaga Bay received an earlier alert.
Some called communication from local authorities after the earthquake slow, and a "system failure", while others say they were left in the "grey" not knowing if they should evacuate.
Others on social media said they were lucky to have had family who phoned to alert them, and friends from overseas.
Commentator Paul Brislen, former Telecommunications Users Association (TUANZ) chief executive, said the alerts could be improved but were largely sensible.
"They only alert you if you're in an area that's affected, and that's really important in New Zealand."
After the Kermadec quake, an alert in the Bay of Plenty urged people east of Matata to evacuate due to expected inundation.
West of Matata, the alert warned of strong and unusual currents, similar to an alert Aucklanders received.
Brislen said degrees of imminent risk from natural disasters and hazards could vary by location, so geo-targeting alerts made sense.
He said Civil Defence authorities had the system "down to a fine art" but some messages probably had too much text.
"You really want either less text or the message to stay there once the sound's gone away, so you can read it."
A Herald reader asked why alerts on certain phones, including some entry-level android models, are voiced in an American accent.
"It's the phone itself, rather than Civil Defence," Brislen said. "It's not like they've decided what flavour voice you get."
Brislen said Apple now allowed some users to change the accent and gender of the Siri virtual assistant's voice.
Brislen said factory settings might also explain why some Kiwis get US presidential alerts.
"What I suspect is happening there is the parallel-imported phones have an American IMEI number and the American system pushes out the message."
The IMEI is the International Mobile Equipment Identity, a 15-digit number unique to each phone.
For some alerts, macrons in Te Reo Māori place names showed up as question marks, so Whakatāne read as Whakat?ne.
National Emergency Management Agency (NEMA) spokesman Anthony Frith said three things dictated who got the alert, and who did not.
Phone compatibility is one factor. The 1990s-style bricks might not get the message but the vast majority of working phones receive the emergency mobile alert.
Cellphone coverage plays a role. If you're off the grid, you don't get the alert, unless you enter a coverage area during the alert's live broadcast time.
That time can vary by hazard, but ranges of one to 12 hours have been used before.
Geo-targeting is the third factor.
It was decided not to send an alert for a beach and marine advisory after the quake off the east coast of the North Island at 2.27am.
The first alert message was only sent to those most at risk of tsunami, from Cape Runaway to Tolaga Bay, NEMA director Roger Ball told RNZ.
The agency decided against sending a beach and marine advisory about surges or currents across a wider area at that time, in places where there was not a threat to life.
Frith said the alert system worked successfully.
"There's always ways we can improve or things that went well that we want to make sure we repeat," Frith said.
After the risk was assessed and the message composed, a trained person would prepare to send the message.
With a mouse click or the push of a button, the alert would go.
Only police, NEMA, the fire service, the health and primary industries ministries, and Local Civil Defence Emergency Management Groups can issue alerts.
Frith said these agencies must abide by protocols around sending alerts, and people had to be trained in these protocols and in how to send alerts.
- additional reporting: LDR