Authorities warn people constantly about the dangers of floodwater during times of wild weather, and John Bagnara knows firsthand why they're so exasperated.

Twenty-six years ago, a 16-year-old Mr Bagnara jumped into a flooded drain in Canberra to rescue another teen caught in the current.

They nearly died.

Despite his fitness at the time, Mr Bagnara could not have prepared for the strength of the rushing water.

"It was like being pushed by a scrum packed by the Brumbies," he said.

Mr Bagnara said despite the time that had passed, he remained vividly aware of the terror of being trapped in floodwater and how lucky he and the boy he jumped in to rescue were to get out alive.

"We were at the mercy of those waters and fortunate to get ourselves out," he said.

The drain filled after heavy rains, creating a current that carried debris like branches and even shopping trollies.

"A current like that … creates a sort of somersault effect where the person trapped in it constantly does underwater somersaults," he said.

He said at the time there wasn't much time for risk assessment, but once the adrenaline wore off, shock kicked in.

"Your legs were swept from underneath you from the force. Your chest was compressed," he said

"Now I can just look back on that and assess just how lucky we were to survive that."

Mr Bagnara received a bravery award for his actions. ( Supplied: John Bagnara )

He said his message was to not underestimate the danger of floodwater.

"It's something that people ... don't understand until they are put into that situation," he said.

"Even in just a few feet of water, cars can get wiped off the side of the road.

"Don't go near it. It's as simple of that. It might delay your journey but you'll get home safely eventually."

Authorities exasperated by 'boofhead behaviour'

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 55 seconds 55 s Drivers rescued and roads wash away in Sydney wild weather

Over the past four days, Sydney has experienced its wettest period since 1990, with 200 people rescued after driving into floodwaters.

NSW Emergency Services Minister David Elliott called out the "boofhead behaviour" of drivers on Sunday and hinted at severe penalties for those breaking the rules.

"We have 400 SES volunteers trying to complete the backlog [on Sunday] … and unfortunately this has included 150 flood rescues," Mr Elliott said.

"That says to me the community is just not taking seriously some of the warnings that have been issued," Mr Elliott said.

"With this sort of boofhead behaviour, we're not going to tolerate it as far as the police are concerned.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 24 seconds 24 s The Parramatta River overflows as floods grip Sydney.

"We have emergency services deployed during a difficult job under difficult circumstances.

"For them to have to divert their resources because people want to hand it out at a time when families are at home worrying about their wellbeing, and certainly the families of the emergency services workers are worried about their wellbeing, it's just not good enough."

Meanwhile in Queensland, the SES dealt with 32 callouts with swift-water rescues over the weekend, as heavy rain and severe storms caused flooding in parts of the south-east and Darling Downs.