Kangaroos, already the cause of the vast majority of vehicle collisions in Australia, are too much for Volvo's self-driving prototypes to handle. 

Engineers at the Swedish auto firm have come up against a marsupial-shaped hurdle, with their self-driving detection systems finding it hard to cope with kangaroos' hopping motion. 

"When it's in the air it actually looks like it's further away, then it lands and it looks closer," Volvo Australia's technical manager David Pickett told the ABC.

Since the cars use the ground as a reference point, the kangaroos' hopping makes it difficult for the car software to calculate how far away the animal is. 

To do so, engineers will need to program self-driving software to identify and recognise a kangaroo when one appears. 

"We identify what a human looks like by how a human walks, because it's not only the one type of human — you've got short people, tall people, people wearing coats. The same applies to a roo," Mr Pickett explained. 

Australian motorists collided with kangaroos more than 16,000 times last year. Volvo expects to have driverless cars available for sale by 2021.