A Victoria Police social media post reiterating the illegalities of using a mobile phone to pay for an order in a drive-through restaurant has sparked a backlash online, while a SA lawyer admitted the law was struggling to keep up with new technologies.

Key points: Take-away drive-throughs are public and road rules apply

Take-away drive-throughs are public and road rules apply Handheld mobile phone use banned while engine is running

Handheld mobile phone use banned while engine is running Legalities on using mobile phones for GPS is complicated

Victoria Police inflamed public outrage on Friday when they posted a social media quiz to illustrate that a fine of $484 and the loss of four demerit points could be imposed on motorists who used a phone to pay for food from a car.

"If you intend to use your mobile phone to pay at the drive-through window, apply the hand brake, switch the engine off and then access your mobile phone," Victoria Police wrote in a social media post.

"In doing so, you are not considered driving."

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It subsequently drew widespread condemnation from social media users, who accused Victoria Police of "revenue raising".

"You can't seriously argue that using a mobile phone to pay for the food is too dangerous, but leaning out of the car window (often with both hands) to collect your food, drinks, whatever is fine," one user replied.

"Victoria Police [this is] absolutely ridiculous and goes against any sort of logical argument one could make," posted another.

South Australian traffic law specialist Karen Stanley told ABC Radio Adelaide's Mornings program that road rules were applicable Australia-wide, with a few slight variations from state to state.

"The road rules define a road as a public area where people drive motor vehicles," she said.

"So even though it [a drive-through] is privately owned, it's still considered a public area because part of the business of the takeaway drive-through is that public cars come in and drive through and buy food."

SA Police said a car would have to be in a "condition where it cannot move by itself" before a handheld mobile phone could be used in such a scenario.

Inspector Cynthia Healey recommended that the handbrake be used and the car switched off.

"A drive-through, as well as car parks, is considered a road-related area as it is an area open to public which is used for driving or parking vehicles," she said.

"Therefore, this rule applies when using mobile devices to pay for food at drive-throughs."

Laws struggling to keep up

Ms Stanley said the laws were designed to stop motorists using their phone while they were just sitting at a red light but were complicated and had "struggled to keep up with technology".

She said the wording of the law was important, because a phrase saying the vehicle needed to be parked "was not really well defined", as well as words that allowed a phone to be used for specific circumstances if it was "securely mounted".

"But clearly you can't be paying it it's securely mounted in your car," Ms Stanley said.

Using mobile phones for electronic payments is becoming increasingly popular. ( ABC Radio Adelaide: Malcolm Sutton )

She added that she had never been approached for help by anyone who had been fined for using a phone in such a situation or, for that matter, in a public par park.

This suggested police were using commonsense and discretion on the matter.

But Ms Stanley said another "tricky area" was the subject of using a phone as a GPS.

Motorists were allowed to use their phones as a GPS provided it was securely mounted to the vehicle.

If, on the other hand, the motorist was not looking at the phone but only using its digital voice to listen for directions, Ms Stanley said it could be permissible if it was not mounted, but as soon as anything lit up on the phone, it became a "grey area".

"The safest policy to adopt is, you can't do anything with a mobile phone unless it's securely mounted or you're parked," she said.

"Except for P-platers," Ms Stanley said.

"P-platers can't use phones at all in their car, in any circumstances."