A significant thunderstorm and rainfall system is evolving across southern and eastern Australia with the potential for widespread flooding in parts of Victoria and southern and central NSW to begin the summer months.
Bureau of Meteorology senior forecaster Scott Williams said north-east Victoria could expect 300mm between the night of December 1 and December 2 and that it could result in the biggest rainfalls in Victoria for 30 years.
While southern parts of NSW could see falls in excess of 100mm on Friday and Saturday with some places seeing double that figure.
"We are talking rain rates in one hour of 30-50mm starting in the west of the state in a fairly limited way...(and then) really exploding across central Victoria and into the east from Friday night to Saturday night," he said.
"The most likely period of flash flooding and widespread heavy rains is Saturday morning including in the early hours... We could see six hour rain rates of 100mm over several parts of the state."
The upcoming weather event is caused by a combination of tropical moisture and a deep low pressure system.
"What we are likely to see is not just this tropical moisture but the development of a low pressure system over Victoria on Friday and into Saturday morning,"
Mr Williams said the system would be coming from the west of the state.
"(This system) coming from the west will lift this tropical air and as a result produce wide spread thunderstorms and widespread and locally very heavy rain."
A flood watch has been issued for the Broken River, Broken Creek, Seven and Castle Creeks and the Goulburn River.