Tonga declares state of emergency as Cyclone Gita approaches

Tonga has declared a state of emergency as it braces for Cyclone Gita's impact. 

Acting Prime Minister Semisi Sika declared the state of emergency at 10am on Monday, hours before the tropical cyclone was due to hit the country.

NIWA says the "dangerous" storm is strengthening and will likely be updated to a category 5.

Cyclone Gita has the potential to wreak serious havoc in the Pacific Islands, but it's too early to say what it might do in New Zealand, NIWA says.

Meteorologist Chris Brandolino told The AM Show on Monday morning that there could be "troublesome issues" here if more heavy rainfall hits following the weekend's deluge.

But he says Tonga is the country we should worry about first and foremost.

"Tropical Cyclone Gita is a category 4. It is forecast by the Fiji Metservice to become a category 5, and is likely today to have a direct impact with Tonga," he said.

"Oftentimes when you have cyclones of this magnitude and construction that's not maybe built the best, you can really see some bad pictures coming out of those areas.

"Unfortunately, it looks like that may happen."

Mr Brandolino says while we often get fixated by wind speeds in catastrophic weather events, it's actually water that's the most serious hazard.

"[The danger is] either from excessive rainfall causing flooding, storm tides or storm surges - which is basically all the water coming from the ocean onto the land because of the cyclone."

Mr Brandolino says New Zealand needs to "put the brakes on" before worrying about the cyclone lashing New Zealand, as there is so much uncertainty as to whether it will even reach us.

"There's been some reports of this and that happening, but it's way too premature," he said.

"We'll be watching it, it's going to go south towards New Caledonia and make a left-hand turn towards the Tasman most likely.

"If anything is to happen here, it won't be this week."

However he says even if we just get some of the residual effects of Cyclone Gita next week, it may cause serious damage.

"What I'm worried about is that we got the heavier rain over the last couple of days, [and] if we do get another impact from an ex-tropical cyclone, that could set us up for some more troublesome issues," Mr Brandolino explained.

"This heavy rain is the primer, and if we see the rain sometime over the next week, it has an accumulative effect.

"[But] the last week of February is looking much better - once this cyclone scenario works its way out of the system later next week and beyond, we're in a much better, happy spot."

Mr Brandolino said the weekend's rain was fuelled by hot air originating from the Tropics.