Police are investigating whether bones found at a block of units at North Bondi belong to a human.

A construction worker was clearing out rubbish from a cavity at the back of a garage when he made the discovery.

The 23-year-old alerted his colleagues, who were working inside one of the units, before calling triple zero.

Forensic police taped off the entrances to the cavity before collecting bags of evidence from underneath the building's stairwell.

The construction worker, who did not want to be named, told the ABC that the bones looked like a hand or a foot.

"I was clearing out rubbish and then came across this hand or foot — it was hard to know what it was, but it looked like it was from a human," he said.

"It was really shocking — I've never seen anything like that before. It made me feel sick."

The bones were sent away for forensic examination, but police are treating the discovery as suspicious.

Anyone with information is being urged to contact Crime Stoppers.