There have been complaints on social media from people who failed to receive a Civil Defence alert on their mobile phone last night - or a fresh alert sent this afternoon.

Civil Defence takes another stab at mobile alert after 1.30am bungle

But a spokeswoman for the Ministry of Civil Defence and Emergency Management says there was not fault with the system, which is designed to push a text message, and accompanying screech, to every phone in a designated disaster area.

Spark, Vodafone and 2degrees had confirmed the alert was successfully sent. There was an issue with one cellsite, but overlapping coverage meant it should not have prevented anyone from receiving the txt.

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Rather, it was a case of not every phone being new enough to receive the alerts.

She said a Civil Defence test in November last year found that around 60 per cent of phones could receive the alerts - roughly double the previous year. The agency expects the percentage to keep increasing as people upgrade their phones (see a list of compatible phones here).

The system is designed to complement efforts to reach people through traditional media, social media and other means.

Some were curious about why an alert would be sent mid-afternoon on the second day, by which time most of the smoke had dissipated. Auckland man Toby Manhire, who did not receive the alert until 8.45pm, tweeted, "I think they must be typing these out one by one and I'm rock bottom of the list, can someone please phone me when the tsunami comes?" One CBD local told the Herald he received the yesterday's alert, but not today's.

The Civil Defence spokeswoman said some people did not receive one or both of the alerts because they were not in the alert area, which in rough terms was bordered by Spark Arena in the East, Victoria Park in the West, the Viaduct Basin in the North and spaghetti junction in the south.

One office worker said he got the alert multiple times. Another told the Herald he was curious that his alert was headlined "Presidential Text Alert" (as similar alerts are designated in the US).

Today's alert
Today's alert

The Civil Defence spokeswoman said this was an issue with phone settings that the agency could not control.