Australian police are searching for clues and possible arsonists as the death toll from the nation's worst bushfires looks likely to top 200.

Victoria state Police Commissioner Christine Nixon vowed to catch anyone who started a blaze as she launched the nation's biggest arson investigation, dubbed 'Operation Phoenix'.

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Police say the bushfires, which swept through Victoria state on Saturday night, are suspicious as there were no reports of natural events such as lightning that would have sparked the blazes.

State Premier John Brumby said anyone found guilty could face manslaughter or murder charges.

The disaster area, encompassing more than 20 towns north of Melbourne, has been declared a crime zone by officials.

At least 173 people have been confirmed dead in the fires, but officials say the toll will rise.

'There are still a large number of people, in excess of 50 ... who the coroner believes are already deceased, but are not yet identified,' Mr Brumby told reporters. 'This is going to be a significant number, it will exceed 200 deaths.'

About 25 fires are still burning in Victoria, with a dozen towns placed on alert as strong winds flared.

'The fires are nowhere near controlled for people to let their guard down,' said emergency official Kevin Monk.

Prime Minister Kevin Rudd (right) said: 'We are left speechless at the thought and the possibility that some of these fires may have been deliberately lit.Kevin Rudd

'Something which the nation must now attend to as a matter of grave urgency is the problem of arson.

'This is simply murder on a grand scale. Let us attend to this unfinished business of the nation and come to grips with this evil thing.'

Taoiseach Brian Cowen has sent a letter of condolence to Mr Rudd after the devastating fires.

Mr Cowen wrote that the loss of life had 'shocked and saddened' the people of Ireland, who shared the pain and sorrow of Australia.

He sent condolences to all of those who have been so tragically bereaved and wished the injured a full and speedy recovery.