The tsunami alert system, designed to warn 15,000 homes and businesses in a matter of seconds, was approved by North Shore City Council and Civil Defence in preparation for the threat of a tsunami.

Residents living in coastal areas identified as warning zones will receive a letter next week inviting them to join in the city's new high tech telephone alert system – believed to be the first of its kind in New Zealand.

North Shore Civil Defence manager David Keay said he was looking forward to the implementation of the alert system, initially for the city's most at-risk areas.

"We're one of the first in this country to implement a telephone-based tsunami alert system, an invaluable tool for our unprotected coastline and position on the earth's "ring of fire".

"We see this system as the best fit for our landscape and are confident that all residents in high risk zones will appreciate the importance of an alert system that could save lives."

More than 15,000 homes and businesses in the most at-risk areas will be invited to opt in to the alert system, either by filling in their contact details online with an individual code, or returning the slip provided on the letter.

North Shore City's Actionline call centre manager Ann Hollway said the warning system technology could handle a large volume of calls in the event of a tsunami warning.

"We tested the system last year on a small scale and it was highly efficient.

"We will soon be ready to test it on a larger scale and we urge all households requested to opt in by June.

"Because no one knows when a tsunami may strike, our best defence is to be prepared and that involves testing the system with as many participants as possible."

When a tsunami is declared by National Civil Defence, the system is activated to automatically send telephone calls to those listed on the alert database.

The automated voice message will announce a Civil Defence tsunami alert and the listener will be directed to immediately tune into Newstalk ZB Auckland, or Radio New Zealand, for emergency instructions.

The alert message will also advise users to avoid using telephone networks to prevent overloading.

No national warning system has been recommended or is operated by the Ministry of Civil Defence.

Details provided to the council when opting in to the warning system will be used for Civil Defence purposes only.