An Adelaide driver has been fined for squeezing flat pack furniture into his car.

South Australian police said the driver and passenger had their heads resting against the unrestrained box when officers stopped the car on Brighton Road at Glenelg East.

Police said both people in the car could have suffered serious injuries or worse had the vehicle needed to make an emergency stop.

The headrests on the front seats had been removed to make more space for the flat pack, officers said.

The driver was issued with a fine for breaching load restraint requirements for light vehicles.

The expiation notice involves a $210 fine and further $60 victims of crime levy.

Police said most accidents stemming from wrongly-secured loads happened at low speeds and often after only a short distance.

"A heavy load is just as likely to fall off as a light load [as] the same G-forces are acting on both," police said in a statement.

"The weight of the load alone cannot provide enough friction to restrain it during normal driving.

"Additional restraint must be used."

A load restraint guide from the National Transport Commission said a load needed to be secured to withstand forces of at least:

80 per cent of its weight in a forward direction

80 per cent of its weight in a forward direction 50 per cent of its weight sideways or to the rear direction

50 per cent of its weight sideways or to the rear direction An additional 20 per cent of its weight vertically.

Police said ropes were extremely ineffective for restraining loads and the flat pack should have been carried in an appropriate vehicle, not the cabin of a passenger vehicle.