"The sustained maximum winds near the centre is about 220km/h, with momentary gusts up to 300km/h."
It is forecast to lie about 475km northwest of the country's capital city - Nuku'alofa - by 1pm today.
The public is being warned of "destructive storm-force winds", heavy rain and thunderstorms with very rough to high seas.
There are also strong-wind warnings, heavy rain and flash flood warnings; as well as a "heavy damaging swell" warning for all of Tonga's coastal water areas.
New Zealand weather experts are also keeping a close eye on the cyclone's movements, as there is still a chance it may affect New Zealand in the coming days.
'Still very powerful'
WeatherWatch says the tropical cyclone has weakened slightly and is now a category 3 strong event.
"Still very powerful, deadly and dangerous. But this is some improvement for those in Yasa's path - Fiji, Tonga," a Twitter alert said.
The MetService said although Yasa was now moving away from Fiji, severe weather is still expected in parts of Fiji.
Meteorologist Tahlia Crabtree said it will start moving south over the next 24 hours and was on track to continue that way - towards the ocean, where it looks to weaken.
As it gets further south, it will lose its tropical cyclone status, Crabtree said.
Another weather system is forecast to arrive over New Zealand next week - bringing heavy rain and strong winds to some areas on Wednesday and Thursday.
