A potato struck a car windscreen after it was fired from the side of Adelaide Avenue by a group with a large stockpile of spuds.

Josh Delaney was driving his Toyota Landcruiser towards Parliament House at 2.30am when his windscreen was struck and cracked by a potato.

He said it was "just beyond belief" that people were shooting potatoes directly at cars.

An hour earlier in the same area, another motorist said his car was hit by what he first thought was an apple. Chris Ciantar said, "I heard a very loud bang and thought I’d hit a small Roo but discovered substance on my car when I arrived at work."

Mr Ciantar, a security guard at one of the diplomatic premises in Canberra, said there wasn't any damage to his Toyota Corolla. The object hit the driver's side door just above the window. "If it was another inch lower, it would have smashed the window," he said.

Mr Ciantar said it was pitch dark and he hadn't seen anyone fire the projectile.

The fear is that what may be a prank, possibly done by kids, will lead to a serious crash if a windscreen is smashed.

Police said they had attended the incident, and the matter was currently under investigation. Anyone who knows anything relevant should get in touch with them.

The mystery is why a person or persons would have bags of potatoes and a spud gun in the middle of the nation's capital in the middle of the night.

The latest incidents come after Danny McCormack was hit in the groin by a potato fired from a makeshift cannon last week while he was standing on the side of roadwork at William Hovell Drive in Belconnen.

Mr Delaney, who was driving to work, first thought a bird was coming towards his windscreen before realising something had been intentionally fired at him.

The potato hit the join between the windscreen and its frame, causing large cracks through the glass.

"If it had have been someone else, a littler car, [the car] probably would have shot off up the gutter and had a pretty nasty accident," he said.

Mr Delaney said he was lucky the potato didn't hit his windscreen higher, as it could have shot right through the glass towards his head.

He said three people came on to the road after he pulled over and collected potatoes from the Adelaide Avenue.

"It's pretty game really, all the protective service cars that float around with the embassies," he said.

Mr Delaney, who reported the incident to police immediately after he was hit, went back to see where the potato had been fired from and found eight bags of unwashed potatoes, several milk crates and empty bottles of beer and vodka.

Mr Delaney's four-wheel-drive was hit from the Yarralumla side of Adelaide Avenue between the Novar Street underpass and the Houpeton Circuit overpass.

Some of the plastic potato bags looked like they had been weathered which suggested this wasn't the first time potatoes had been fired from the spot, he said.

Potato cannons are generally home-made devices constructed from plastic pipes and rely on igniting spray from aerosol cans, such as deodorant or hair spray, to propel projectiles from the tube.

Mr Delaney said the more he thought about the incident, the angrier he became.

"It's not funny. Like, I'm pretty laid back, but the more I think about ... they'd been out there probably for a couple of hours. Just crazy."