The current official death toll is 181, but Victoria State Premier John Brumby has warned that dozens of bodies remain unidentified, with others unrecovered, and the final toll is likely to exceed 200.
Ms Nixon said rescuers were carrying out a thorough search of charred areas.
"Road by road, house by house, we are working our way through," she said.
"As we go into areas now that we haven't been able to get into before, we believe we'll find many more."
HOW BUSHFIRES SPREAD
1 Fires start in hot dry windy weather 2 Embers blown ahead of fire front 3 Spot fires start where embers land
US President Barack Obama was among a number of leaders from around the world who called Mr Rudd to offer condolences and help.
Talk of arson in the town of Whittlesea, north of Melbourne, provoked immediate fury, reports the BBC's Phil Mercer.
The thought that some of the devastation was man-made is, to many locals, incomprehensible, he reports.
A record heat wave and changing wind direction on Saturday helped fan the flames.
Many people are searching for loved ones among the survivors taking refuge in Victoria's relief centres.
Relatives concerned about family members in the affected areas can contact the Australian Red Cross 24-hour helpline on (0061) 393283716 or, from the UK, the British Red Cross international tracing and message service on 0845 053 2004.
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