Engen Birchleigh North *****ing us

Engen Birchleigh North is doing something very cunning. Every time a person buys petrol, their pump always adds 5c or 7c. When we ask the petrol attendants why it is so, they say the manager/owner disabled the keypads so as not to put the exact amount but to pour petrol the old traditional way. When you object , you have to get out of the car and go and pay at the garage window. So he pockets all these customers' 5c, and makes how much extra a day out of that ? Is this even allowable by law?