New South Wales police have been opening cars and checking for valuables as part of a crackdown that invokes an obscure road rule that results in people being fined for forgetfulness.

Yes, people in NSW can be fined $108 for leaving their car unlocked.

The obscure road rule is rarely enforced, but police north of Wollongong have been reminding residents of it by checking car doors in suburban streets.

"We have been seeing how many cars are unlocked and then seeing what valuables are inside," Detective Inspector Brad Ainsworth said.

"Unfortunately it was pretty disappointing. We found 30 vehicles over a couple of nights unlocked and with property in them."

Instead of fining the cars' owners, police have been knocking on the door of residences or placing a warning note on car windscreens.

Wrong thing to do, civil liberties president says

The patrol has angered NSW Council for Civil Liberties president Stephen Blanks.

"I'm really concerned about police opening cars and having a look in and going through the contents," he said.

Stephen Blanks says the unlocked car law "borders on stupid". ( ABC TV )

"Police should not be doing that kind of thing without a warrant, and the mere fact a car is unlocked is not a cause for suspicion there's something illegal in it.

"If they do that kind of activity they will lose community support and they already require strong community support to do their job well.

"This kind of activity is just the wrong thing to do."

Detective Inspector Ainsworth has defended the decision, saying police have a duty to proactively prevent crime.

He said by highlighting potential opportunities for criminals, they were stopping a break-in before it happened.

"There have been lots of steal from motor vehicle offences in the [Illawarra's] northern suburbs and by going out and having a look, we're more than justified in doing that and unfortunately it's proven right.

"Cars are in their driveway and on streets and people leave their wallet in there, credit cards get stolen and used with PayWave and the police are chasing up those things, CCTV [footage], and it creates a lot of work."

Fining people a 'last resort'

While the fine is available to the police to issue, Detective Inspector Ainswoth said they would rather not use that option.

Police have been leaving warning signs on windscreens of unsecured cars. ( ABC Illawarra: Justin Huntsdale )

"It would be a crook day when we have to do that, but that's one of the options up our sleeve," he said.

"We've done community engagement in the northern suburbs and residents say they don't have to lock anything, but times have changed and offences like breaking into cars is happening.

"We have to safeguard against it and the owners of the property have to safeguard against it too."

Mr Blanks said the rule against leaving cars unlocked was "bordering on stupid".

"Governments have to be careful about what they criminalise and impose penalties for, and it's very important that laws have community support and are not stupid," he said.

"The idea police are going to walk along the street testing cars to see if they are unlocked makes people uneasy."

Residents don't appreciate police actions

Commenters on ABC Illawarra's Facebook page said car break-ins were happening regardless of whether people locked their vehicles, and raised concerns about civil liberty breaches.

"I have spent $1,000 in the past replacing broken windows after thieves have broken into my cars, which have never had a single coin or anything of value," Belinda Blackmore wrote.

"This is the nanny/police state we live in now … how dare NSW police randomly check people's cars," Jeremy Boulton wrote.

"It doesn't belong to them or the state … don't touch my things. If I forget to lock my car I'll be responsible if I have something stolen."