Melbourne's beaches are likely to be off-limits and unsafe for swimming until at least Wednesday, when temperatures are set to soar into the mid 30s.
Port Phillip Bay's 36 beaches have been unsuitable for swimming since Monday, after Sunday night's giant downpour washed faecal matter and other pathogens into the bay.
The situation was unchanged by Tuesday, after the Environmental Protection Authority downgraded the water quality rating to poor on Monday morning when a month's worth of rain fell on certain parts of the state.
EPA group manager of applied sciences Dr Anthony Boxshall warned that beachgoers were at risk of both infection and gastroenteritis and said the situation was unlikely to improve until Wednesday afternoon.
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It's bad news for beach lovers as recent cool conditions are expected to make way for summer temperatures later in the week, with the weather forecast to hit 34 degrees on Wednesday and 36 on Thursday.
"I strongly expect that we'll be forecasting every beach as poor water quality again this afternoon and tomorrow morning," Dr Boxshall said.
"But the weather does change, that's why we do twice-a-day forecasts. We'll update that this afternoon and before 10am tomorrow."
From human faeces to animal faeces, everything from both the pavement and overflowing sewer systems are sent into the bay each time heavy rainfall hits the state.
"It's gastro that we're worried about and infections. If someone had an open wound on their hand, you can get pathogens."
Dr Boxshall said it generally took 24 to 48 hours for pathogens to dissipate after a storm but predicted some outer beaches could return to health in time for Wednesday's heat.
"We usually find that there are beaches that bounce back faster and it's usually related to the fact that they're not right next to the Yarra River or Werribee or a stormwater drain."
Fortunately, Wednesday's warm weather is expected to improve the situation, with sunlight a key to good water quality.
"Sunlight breaks down the bugs," Dr Boxshall said. "They don't like it. Sunlight and wind are the friends of good beach water quality and rain is the enemy of good water quality."
EPA scientists will spend the next 24 hours cultivating a water sample from the bay to identify what kind of bugs are swimming in the bay.
"They have to actually culture the bugs and bacteria in a lab and then they grow them and check and tell us how many there are," he said.
"It'll take a while for them to grow but we will have a preliminary result in the morning. I imagine because of that we'll see some beaches will be upgraded and that will be by about 10am."
Outer coastal beaches such as Lorne and Lakes Entrance bounce back much quicker than inner-city beaches and were safe for swimming.
Dr Boxshall encouraged Victorians to consult the EPA's twice-daily water quality updates before swimming.
The warning comes just four weeks after Melbourne's beaches were last contaminated with faecal matter, with 21 beaches around the bay, including popular spots St Kilda, Port Melbourne and Elwood, declared unsafe for swimming.