Tropical cyclone Pam has been upgraded to category 3 and weather experts fear it could reach supercyclone levels as it slowly moves towards Vanuatu.

Vanuatu remains on alert as the potentially devastating weather system moves almost due south at about 10km/h.

On its current path, the cyclone is predicted to pass between Vanuatu and Fiji, although it's likely to be closer to Vanuatu, according to Fiji forecasters, NaDraki Weather.

Further intensification is anticipated over coming days and NaDraki says computer models predict it could be upgraded to category 4 or even a category 5 supercyclone.

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Its intensity is expected to peak on Friday as it passes well west of Nadi.

"Rain remains the most significant threat to Fiji we believe, with showers increasing today and tending to rain tonight and becoming heavier tomorrow," the latest NaDraki update says.

"We could see periods of heavy rain on and off through to Saturday and possibly even Sunday so if you are in an area known to be prone to flooding please review your options and make preparations now."

Tropical cyclone warnings are in force for the Temotu, Malaita and Makira provinces in the Solomon Islands, with heavy rain, gale force winds and high seas expected over the next 48 hours.

WeatherWatch says the storm is likely to narrowly miss New Zealand.

However, swells and rips are likely to be very dangerous around North Island eastern and northern beaches.