Parts of Melbourne were battered with up to 58mm of rain in the first two days of October - more than 80 per cent of Victoria's average rainfall for the month.
The onslaught marked Melbourne's wettest October day in 16 years, with the city soaked by up to 25mm of rain within an hour on Thursday afternoon.
About 7000 homes were without power overnight on Thursday and more than 270 calls for help were made to the State Emergency Service as overwhelmed drainage systems triggered widespread flash flooding.
More than 100 of the calls for assistance were for building damage, almost 70 for flooding and more than 70 for fallen trees.
The worst hit areas appeared to be spread across metropolitan Melbourne, including Toorak, Pakenham, Reservoir, St Albans and West Melbourne.
"We have assisted Victoria Police with six water rescues, vehicles getting into trouble due to entering floodwater or driving in roads that had water catching due to the rainfall that fell pretty quickly in a short period of time," SES duty officer Jessie Schleibs told AAP on Friday.
Prolonged rainfall continued across Victoria on Friday, particularly in central and eastern chunks of the state.
Catchments in the Yarra Ranges recorded the heaviest totals, with Coldstream logging 33.2mm since midnight on Friday, while Melbourne's central Olympic Park gauge recorded 13.4mm.
South Australia also had significant rainfall, with 20mm-25mm falling across parts of the Adelaide Hills in 24 hours, while Victor Harbor was battered by 23mm since 9am on Friday.
A reprieve from the deluge in Melbourne on Friday afternoon is expected to be short-lived, with the downpour again ramping up early on Saturday.
Elsewhere, slow-moving storms are expected to develop in the Mallee and eastern South Australia on Friday evening, while heavy showers, damaging winds and large hail will continue through Gippsland continue on Saturday
Cumulative totals across Gippsland and the Otway coast could surpass 100mm by Sunday evening, with severe weather warnings in place in those parts of the state.
Flood watches are current for catchments across much of the central and eastern parts of the state, and a minor flood warning is in place for the Bunyip River east of Melbourne.
"There is much more to come," Bureau of Meteorology senior meteorologist David Crock said
Conditions are expected to ease from the west on Sunday as the low pressure system moves offshore.
Thunderstorms are forecast across the ACT and in southern NSW from Friday afternoon, bringing heavy rain, damaging winds and large hail across the weekend.
"By the time we get to Sunday, there'll be some lingering showers and perhaps some storms, mainly through Gippsland into the south coast of NSW," Mr Crock said.
Melbourne, Sydney, Canberra and Adelaide should expect mostly dry conditions by Sunday, he said.
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