A disgruntled Sydney resident has revealed why she copped a $112 fine as she broke a road rule she didn't know existed.

She lives in the Sutherland Shire in NSW and received an infringement notice for leaving her car window down just after 7:40 am on August 4th.

“$112 fine for not shutting car windows, the car was parked on the street at my home and husband was working in garden in eye view of the car,” she wrote on Facebook.

“I never knew this was a crime so shut your windows.”

The penalty notice labels the offence as "not lock doors secure windows (vehicle unattended).

The NSW road rule book states that a car must be secured when a driver is more than 3 metres away, which includes closing and locking the doors.

A window opening of less than 2cms is allowed, but many are unaware of the rule.

Facebook friends commented in shock.

“What a joke!” One person said.

“Seriously?!” Another wrote. “Gosh Australia has really silly fines for everything.”

“Clearly revenue raising,” a woman said. “The officer should have issued a warning if you were present.”

The rule has been slammed as "unbelievable".