Tropical Cyclone Rene, which is now listed as Category Four, is now hitting Vava'u and Ha'apai, islands in Tonga's north.
Tonga police say there is no communication with Vava'u at the moment and the lights are out.
The cyclone had been predicted to cause severe destruction from early this morning.
The Fiji Meteorological Service says Cyclone Rene is moving southwest at 12 knots towards central and southern Tonga where people have been advised to take precautions.
Tonga's National Disaster Management Office is busy preparing responses throughout the various island groups.
Rene has winds of 90 knots, or nearly 170 kilometres, an hour close to the centre.
With forecasters predicting the cyclone to intensify as it heads towards Tongatapu, the Disaster Management Office's Male'u Takai said they were bracing for the worst.
He also said they were considering evacuation plans in case of storm surges.
Samoa and American Samoa were spared the worst of Tropical Cyclone Rene which passed by on Saturday night.
But one man died in American Samoa after a fall from a building as he prepared for the cyclone.
In other parts of the territory, some trees were uprooted and there was flooding in some streets and slips.
The south east of Samoa's island of Upolu, which bore the brunt of the damage from last September's tsunamis disaster, felt the worst of Rene as it passed by, but damage was limited to a few fallen trees
A tropical cyclone gale warning that was in force for Niue has been cancelled