What is your recommendation/ advise with regards to leaving baby in car while paying for petrol? Is there a specific amount of time they are allowed to be alone in there while mum is lining up at the cash register?

There is no specific time frame that is “safe” for a baby to be left unattended in a car.
Safety organizations have the following information.

Red Nose
Never leave your baby unattended in the car – not even for a short time.
https://rednose.org.au/article/are-there-recommendations-for-car-seat-or-baby-seat-use

Raising Children Network
Never leave your child unattended in a car. On a hot day, the temperature inside a car can get to dangerously high levels and cause serious harm or even death to your child. Read more about heat and cars.
https://raisingchildren.net.au/preschoolers/safety/car-pedestrian-safety/car-safety#car-safety-tips-limbs-locks-loose-items-pets-kerbside-nav-title

Kids Safe
Never leave children alone in vehicles, even if they are asleep and you can see them.
https://kidsafe.com.au/wp-content/uploads/2020/06/202002-Kidsafe-Parents-Guide-to-Roads-Web.pdf