Authorities in Fiji and Tonga are warning locals to be prepared for a tropical cyclone as a storm in the region is expected to intensify.

A heavy rain alert has been issued by the Fiji Meteorological Service; warning people that heavy rain, showers and some thunderstorms are forecast later tonight and tomorrow.

New Zealand's WeatherWatch said it looked like a cyclone - which will be named Tropical Cyclone Tino - would develop fully by tomorrow or Thursday and that Fiji would take a "direct hit" on Friday, before Tonga saw its effects on Friday.

"The storm will still be developing as its moves through the Fiji area - making it a little hard to work out intensity in advance," a spokesman said.

"But it's hoped the storm will likely remain at the lower level of the intensity scale as it crosses land - therefore, rain and blustery winds will be the main concern for all of Fiji."

Torrential rain, gale-force winds and dangerous sea conditions will be the main issues in Tonga by the weekend if the tropical cyclone remains on its current path, WeatherWatch said.

Some of those gale-force winds also had the potential to be damaging.

"The good news is that it is moving through fast - so really, the 24 hours of Saturday look wet and windy in Tonga. But across Sunday, winds fade out, skies dry out and the sun returns."

#Developing: #Fiji & #Tonga brace for direct impact by potential #cyclone, alert for #NewZealand beaches (+9 Maps) https://t.co/SLuLHP614b via @weatherwatchnz #weather

— WeatherWatch.co.nz (@WeatherWatchNZ) January 13, 2020

HOW IT MAY AFFECT BEACH/SEA CONDITIONS IN NZ:

The tropical storm or cyclone activity, at this stage, is not expected to impact New Zealand's weather conditions.

However, systems place it to the east of the East Cape and WeatherWatch said although it may not impact the weather too much, it may make for dangerous conditions at the beach in the east of the North Island.

"Offshore storms like this have proven deadly in New Zealand in recent years," WeatherWatch said.

The storm is expected to be the closest to our shores from Sunday night through to Monday.

"While surfers love these setups, we cannot stress enough how dangerous offshore storms can be for children or weaker swimmers in the surf - even if the weather at your beach may be stunning, sunny hot and calm."