Life Saving Victoria warns swimmers may be less fit and more vulnerable due to the coronavirus lockdown after three separate drowning deaths in one day.
A child is also fighting for life following a waterside emergency in suburban Melbourne.
Victorian Premier Daniel Andrews said on Thursday the three deaths, which occurred on the state's Mornington Peninsula and in the Gippsland region, marked a "tragic day".
"It's a timely, tragic reminder of just how dangerous our beautiful coastline can be," he told reporters.
"I want to send my deepest sympathies to the families involved (and) give a shout out to the surf lifesaving volunteers, SES volunteers and emergency services who were on scene."
A woman in her 20s died after swimming out to rescue a teenage girl struggling in the water at Venus Bay, two hours southeast of Melbourne, about 7.30pm on Wednesday.
The woman set off with several others but got into difficulty before she was pulled from the water by an off-duty lifeguard.
Everyone else made it safely back including the teenager, who was checked by ambulance crews.
About 3.30pm, four people were swept off rocks at Bushrangers Bay near Cape Schanck, close to the southernmost point of the peninsula.
Two members of the public entered the water to assist, police said.
Aida Hamed, a Lalor-based Australia Post worker in her 40s, died and five other people were rescued.
Four of the group were taken to hospital.
"We are deeply saddened at the passing of Aida Hamed," an Australia Post spokesman said in a statement to AAP.
"We extend our deepest sympathies to Aida's familiy and colleagues at this sad time."
Bushrangers Bay Beach is unpatrolled and according to Life Saving Victoria, "moderately safe when waves are low". But it warns swimmers should "stay on the bar and clear of the rocks and rips".
The third death was that of a man in his 80s, brought to shore unconscious at the popular Rye Front Beach in Tootgarook, also on Wednesday afternoon.
All three incidents will be investigated by a coroner.
Lifesaving Victoria general manager Liam Krige encouraged members of the public to swim between the flags and warned against immediately trying to rescue others.
"We would urge all bystanders to, if they see someone in difficulty, dial triple zero and request the correct resources to help," he told ABC Radio on Thursday.
"Don't take the sea for granted - it can be a dangerous, albeit beautiful place."
Mr Krige added that the easing of coronavirus restrictions heading into summer could have contributed to Victoria's record number of drownings in the past six months.
"People probably aren't as fit as they used to be ... their abilities probably aren't what they thought," he said.
Meanwhile, a three-year-old girl was found unresponsive at Lysterfield Lake in suburban Narre Warren about 5.50pm on Wednesday.
She was rescued from the water and taken to hospital in a critical condition.
A police investigation has been launched into the incident.
