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New Zealand is about to install more sea gauges to improve its warning systems for tsunamis.

There is one trial gauge in Wellington Harbour, which successfully detected the Solomons Islands tsunami in April, but Land Information New Zealand (Linz) expects to put two off the coast of Gisborne and Napier soon, pending resource consent decisions.

Linz has identified 20 sites around New Zealand and offshore for the gauges, with the aim of detecting whether an offshore earthquake or sub-marine landslide has caused a tsunami and how big it will be.

"We are well down the planning stages for gauges for the Chatham Islands and the Kermadecs," said Linz's Geodesy manager, Graeme Blick.

The offshore islands are considered priority areas because gauges there will give more warning time to geophysicists.

Linz is working with the Institute of Geological and Nuclear Sciences (GNS Science) to install the gauges, while the National Institute of Water and Atmospheric Research (Niwa) has been identified the specific sites.

The work has come in the wake of the disastrous 2004 Boxing Day tsunami and a subsequent report by GNS Science in late 2005 on the state of New Zealand's warning systems.

While the risk of a tsunami hitting New Zealand has not increased, the Boxing Day catastrophe was a timely reminder that New Zealand's warning systems needed upgrading, authorities said.

Once the gauges are in place, real-time sea level readings will be sent to GNS Science which will analyse the data and alert the Ministry of Civil Defence and Emergency Management of any tsunami threat.

The ministry is responsible for issuing civil defence warnings and organising the appropriate response.

While New Zealand is dependent on overseas bodies such the Pacific Tsunami Warning Centre in Hawaii to provide warnings of long-distance tsunamis, the domestic gauges will enable GNS Science to issue local alerts and track the progress of tsunamis at a local level.

New Zealand co-operates internationally with many countries on tsunami warning and information from its gauges will be relayed to Australia and the Pacific centre in Hawaii.

While new-age Dart (Deep Ocean Assessment and Reporting of Tsunami) sensors are considered beyond New Zealand's financial scope, the country will benefit from Australia's deployment of one in the South-East Tasman Sea - which should detect any movement in the Puysegur faultline off the Fiordland coast.

"We have close working ties with Australia. Some of their most important gauges lie near New Zealand on offshore islands such as Norfolk Island and Macquarie Island. By exchanging data we can ensure that we're not duplicating effort and that we're using all the resources available to us," Mr Blick said.

He said the US is also planning to install Dart sensors off the east and northern coasts of New Zealand.

"We are encouraging them to do that and will look to assist them with maintenance and ship time as we can."

The Dart gauges cost around $500,000 each and are costly to maintain. Mr Blick said authorities here will monitor their performance and might look at them in future, but in the meantime the sea gauges, costing about $100,000 each, are considered more appropriate for New Zealand's needs.

It is hoped to have most of the gauges installed in the next two years.