Much-needed rain to help Victoria's bushfire battle could also pose a threat to Melbourne's major drinking water source.
The state government on Tuesday said it has an order for desalinated water on stand-by as it monitors a fire burning near the Thomson Reservoir, east of Melbourne.
Authorities are concerned fire will enter the catchment of the dam. Subsequent rain would then wash bushfire-contaminated debris into the reservoir, Water Minister Lisa Neville warned.
"Our concern right now, ironically, is if those really big thunderstorms which saw some 30mm of rain fell on the catchment, fell on those fires then we would start to see run-off obviously much earlier than we thought," she told reporters.
If that happens, drinking water could be taken from the bottom of the dam, not the top, she added.
The reservoir is at a 56 per cent capacity, and Ms Neville flagged a "substantial order" from the desalination plant may be needed in 2019.
Subtropical conditions are expected later in the week bringing humidity and rain, but the falls might not be enough to aid the bushfire fight.
Increasing winds and sultry conditions could reignite hot spots across the state, as authorities brace for the prospect of more blazes sparked by lightning.
A watch-and-act message remains in place for Grantville in Victoria's southeast, as fire crews prepare for gusty conditions.
Crews are on watch after lightning sparked multiple fires and air crews will scan the state's northeast region for the rest of the week to detect any new fires.
Smoky conditions are also expected to continue as fire crews continue to keep blazes within containment lines.
"We fully anticipate that additional fires will start during the next few days as a direct result of the lightning," Forest Fire Management Victoria assistant chief officer Aaron Kennedy said.
"Our fire detection towers will be operating for extended hours because of the increased threat."
Earlier in the week the Bureau of Meteorology recorded 18,000 lightning strikes in 24 hours.
There is a potential for thunderstorms across the state on Wednesday and Thursday, with BoM meteorologist Chris Godfred saying there could be heavy rain.
"There could be falls of 50mm falling quickly - within an hour in some parts," he said.
The average monthly rainfall for February is about 40mm.
The weekend is forecast to be dry and calm.
