A crash that seriously injured a teenage boy in Sydney's south-west has prompted calls for motorised pushbikes to be banned.

Pono Aperaham was hit by a van and then reportedly a ute at an intersection in Heckenberg after he snuck out of his grandparent's house a little after 7pm (AEST) yesterday.

The 13-year-old was not wearing a helmet and had no lights on the bike.

He remains in Westmead Hospital's intensive care unit in a serious condition.

Among the first to arrive at the spot where the boy was hit was neighbour and family friend Julie.

"It was quite horrific and not only that, to go see the family and to let them know that the boy had been hit by the car," she said.

"Ban those bikes. Get rid of them. Get them off the road."

Friends of the teenager's grandparents say they only bought the bike for him as a gift yesterday.

Inspector Phil Brooks from the Traffic and Highway Patrol says the bicycles pose a danger not only to those riding them.

"Certainly this highlights the dangers of mountain bikes that have had a motor attached to them," he said.

"You people that are riding these bikes without proper helmets, without proper riding wear, without lights on those bikes do run a very great risk not only to themselves but to other road users.

"With school holidays well upon us we'll be calling for parents and older family members who see these bikes on the road, if their families are involved in the use of these bikes please take action straight away and prevent them from riding those cycles."

The bicycles are currently classified as toys.

Opposition spokesman Walt Secord says the New South Wales Government should consider a ban or tighter regulations.

"The fact is these are not simple bicycles. These are poor man's motorcycles. These are de factor motorcycles put together in people's backyards," Mr Secord said.

The van driver has undergone mandatory drug and alcohol testing.