Australia has begun evacuating thousands of people from its northeastern coast ahead of a cyclone that could rival Hurricane Katrina in its destructive power.

Cyclone Yasi has been upgraded to a Category 4 storm.

Forecasters say it is likely to further strengthen and could generate winds of up to 175 mph before it hits land on Wednesday morning.

An official warning has been issued saying it poses a "serious threat" to communities in north Queensland.

More than 400,000 people live in Yasi's predicted path, which includes the towns of Cairns, Townsville and Mackay.

The region, which takes in Australia's Great Barrier Reef, is a major tourist destination.

Cyclone Yasi is 400 miles wide and bearing down on an area which has already suffered from devastating floods.

The state premier, Anna Bligh, has described it as a "monster, killer storm" and she predicted deadly surges inland.

Sea levels are expected to rise significantly and will lead to flooding of low lying coastal areas according to an official forecast.

Yasi will also bring damaging waves and strong currents.

A strong monsoon is also feeding the cyclone and this could lead to it maintaining strength as sweeps inland, potentially up to 600 miles to the outback mining city of Mount Isa.

Airlines are laying on extra flights to get people out of the area, especially tourists with nowhere else to go.

But the storm is so wide that finding somewhere safe to move to will be difficult for tens of thousands of people.

The holiday resort of Hamilton Island has already been evacuated, and coal terminals in Bowen and Mackay have closed down in another blow to the state's mining industry which has already been disrupted by flooding.

Ms Bligh said the threat to life and property was compounded by the possibility it could trigger a tsunami-like storm surge, especially if its landfall coincided with a high tide.

Police have been empowered to use force if necessary to move people from danger zones.