Lifeguards patrolling the North Island's east coast beaches are warning swimmers to be wary of dangerous rips and large surf as the remnants of Cyclone Waka roll in.

MetService forecaster Allister Gorman said that 2.5m swells began building along east coast beaches north of Auckland and in the Bay of Plenty yesterday and would peak today and tomorrow.

In Whangamata, veteran lifeguards were yesterday being asked to help cope with expected huge crowds and the threat of big waves and rips.

Lifeguard Dylan Lawrence said the club had been ringing former guards to come in and help at the popular beach.

Whangamata yesterday had up to 8000 people at its peak.

Similar swells from Cyclone Waka's wake are expected to hit East Cape, Gisborne and Hawkes Bay tomorrow, as the cyclone, which blasted Tonga on New Year's Day, heads south.

The cyclone tore through the northern parts of Tonga, destroying homes, crops and boats with 6m seas and wind gusts of up to 250 km/h (135 knots).

While 2.5m swells are moderate by west coast standards, beaches such as Mt Maunganui have surf of that size only once or twice a year.

The area lifeguard supervisor for the Mount, Papamoa and Omanu beaches, Brett Girven, said most holidaymakers would not be used to such conditions and the beaches might be closed as a last resort.

Meanwhile, an Air Force Hercules with relief supplies of food and emergency shelter could be on its way to Tonga soon, pending a formal request from the Tongan Government.

The Government's duty minister, Trevor Mallard, said the Government expected to hear from its Tongan counterpart today on what aid was needed in the wake of the cyclone.

Tonga's National Disaster Management Office was expected to meet late last night or this morning to assess damage and determine how much help was required before making an official request.

The kingdom's northern Vava'u island group, a popular tourist destination, was worst affected, with up to 70 per cent of buildings damaged or destroyed.

Unconfirmed radio reports said one person had died in the storm.

Waka formed on Sunday over the French territory of Wallis and Futuna before heading south between Samoa and Fiji and hitting Tonga late on New Year's Eve.

The cyclone has since headed into open sea and has been downgraded to a tropical storm with 100 km/h winds at its centre, about 1200km south of Tonga.

Elsewhere in the Pacific, Vanuatu was rocked by a powerful undersea earthquake yesterday that measured 7.3 and caused widespread damage.