Everyone does this, but few know a $114 penalty can be slapped on them if the police catches them.
NSW Police confirmed with Yahoo Finance that leaving the engine running then walking away more than 3 metres from the car is an illegal act.
Most Australians, especially those with children, would have done this at some stage. But it's against the law.
The offence is listed in the same legislation that also penalises motorists if they walk away from the vehicle with windows open, doors unlocked or with the key in the ignition.
Similar laws exist in other states, with varying financial penalties.
While these rules might seem nitpicky, they are there for legitimate reasons.
"Inviting theft creates unnecessary work for the police," said greeslips.com.au insurance expert Corrina Baird.
"Inviting theft also creates work for car insurers and costs them money – but only if they accept your claim."
According to Baird, insurer Budget Direct revealed 18 per cent of car thefts occur when the key is left in the ignition.
"Every 10 minutes somebody in Australia steals a vehicle and one-in-three owners never see their car again," she said.
"Be more vigilant if you own a Holden Commodore, live in Northern Territory or go out on Friday nights."
Another reason for adhering to the law is if the key is left in the vehicle and it's stolen or damaged, the insurer may have grounds to not pay out your claim.
There will be a clause in almost every policy that blocks payouts made when the policyholder indulges in an illegal act.
"Most vehicle policies state drivers must take all reasonable steps to protect the vehicle from loss or damage," said Baird.
"If your vehicle is unlocked and unattended, with keys in the ignition in a busy petrol station, are you taking a reasonable precaution?"
And if you think the police would not bother with such petty offences, think again.
Sydney man Ben Judd copped the wrath of authorities in January last year when he briefly stopped at a service station to buy himself a meat pie.
Judd had left his windows open and the doors unlocked, and the $112 ticket was written while he was inside the store.
"I was parked inside the petrol station, not on a road... Revenue raising at its finest. Thoughts? Is that even a law?"