States and territories have different penalties for driving with overloaded cars but each include expensive fines as one Victorian p-plater learnt in the early hours of New Year's Day.

The 37-year-old probationary license holder was allegedly driving a black Mercedes in Melbourne's inner south at 1.30am with eight people in her unregistered car.

Police said the front seat passenger had someone sitting on their lap while two other people were balanced on three people sitting in the back seats "all without seatbelts".

The St Kilda woman is expected to be charged on summons for infringements including driving with passengers not wearing seatbelts and having an overloaded vehicle.

She is also expected to be charged with failing to display P-plates, drink driving and using an unregistered car. Police said her Mercedes was impounded for 30 days, costing $950.

The three passengers, who were sitting on the laps of others, were given penalty notices and fined $385 for failing to wear a seatbelt.

Drivers and passengers older than 16 years face fines and demerit penalties in Australia's states and territories for being unrestrained in a car.