Badly damaged buildings are pictured near Vanuatu's capital of Port Vila. Philippe Carillo/AFP

Tropical Cyclone Harold surged back into a scale-topping Category Five storm on Thursday as it sideswiped Tonga after leaving a trail of destruction across the South Pacific.

The island kingdom declared a state of emergency late Wednesday as the storm bore down, warning of destructive winds and massive sea surges.

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Police said early on Thursday that power had been cut in parts of the country and howling winds were gathering pace amid heavy rain after Harold surprised meteorologists with its sudden strengthening.

The cyclone killed 27 people in the Solomons late last week before barrelling southeast to directly hit Vanuatu as a Category Five, obliterating entire towns in the northern provinces.

There have been no reports of deaths in Vanuatu, with emergency workers saying many residents in the hardest hit areas were able to take shelter.

Harold weakened slightly to a still-formidable Category Four as it lashed Fiji Wednesday but hopes the storm was dissipating were dashed as it regathered momentum heading towards Tonga.