If you get an emergency txt from Civil Defence between 6pm and 7pm, don't panic - it's just a drill.

The Ministry of Civil Defence and Emergency Management (MCDEM) is testing a new Emergency Mobile Alert system designed to send a message to cellphone owners nationwide in the event of a disaster. The system can also be targeted to a specific area.

The alert is expected to be received by around half of cellphones, up from around one third in a test last year, due to people upgrading models in the meantime (a list of phones capable of receiving the alert is on the ministry's website here), along with a guide to check your phone is properly setup to receive alerts).

Today's drill follows a bungled test on October 4 last year that saw Vodafone customers received test alerts between 1.30am and 2am.

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Many took to social media to vent about having their sleep interrupted multiple times by repeated txts.

Vodafone huffed in a statement to media: "It is important to clarify the Ministry takes full responsibility for sending this error txt. It was accidentally initiated by one of the Ministry's overseas technology suppliers, and not Vodafone or any of the other ISPs involved in the project."

Civil Defence comms manager Anthony Frith told the Herald this morning there's "no chance" of that happening again.

"That was when the system was still in the hands of the Dutch provider [one2many] that created it," Frith says.

Civil Defence now has direct control of alerts, Frith says. He points out that a test in November last year went smoothly.

Strictly speaking, Emergency Mobile Alerts are not txt messages. They use their own dedicated bandwidth, so they are not affected by any congestion on Spark, Vodafone or 2degrees' networks.

You don't have to sign up for the alerts (nor can you opt out). The alerts are automatically sent to every compatible phone on the three mobile operators' networks.