An off-duty police officer spotted the car in Richmond and couldn't believe what he saw: two people squeezed in the front seat and three more crammed in the back with a child on someone's lap.

The child didn't have a seatbelt or a child seat, police said.

The cop scribbled down the car's registration number to check the driver's details when he returned to work following the 11am incident on January 7.

"Further checks revealed the driver of the car was an Uber driver and he was on an Uber trip at the time of the incident," said police spokeswoman Amelia Penhall.

The driver, a 33-year-old man from Elsternwick, was brought in for questioning.

He had his car impounded for a month due to "overloading" laws.

The man is expected to face further charges on summons for a variety of driving offences.