In NSW it is considered an offence to leave a child unsupervised in a car and can carry a fine of up to $22,000, but if you read the legislation it does leave some room for running quickly into the petrol station and back out again, you just need to judge the situation for yourself at the time to make sure your child won't over heat, be exposed to any danger or become distressed that you aren't there:

CHILDREN AND YOUNG PERSONS (CARE AND PROTECTION) ACT 1998 - SECT 231 Leaving children and young persons unsupervised in motor vehicles

Me personally, i have never/will never leave my kids in the car unsupervised, i just don't think its worth the risk and have always taken the time to get both kids out and put them back in again.