Mysterious black spheres about the size of golf or tennis balls have washed up along the length of Coogee Beach, prompting council to barricade the sand and mount an urgent investigation and clean-up operation.

The “mysterious, black, ball-shaped debris” with “the appearance of dark spheres” were first discovered by Randwick City Council lifeguards on Tuesday afternoon, triggering the closure of the beach.

The council said in an Instagram post on Tuesday evening that its environmental officers had collected samples and sent them for testing to investigate the origin and composition of the material, which could be a mixture of oil and debris.

“At this stage it is unknown what the material is, however, they may be ‘tar balls’ which are formed when oil comes in to contact with debris and water, typically the result of oil spills or seepage,” the council said on Instagram.

The Instagram post shows the balls littering the length of the beach during the last hours of daylight on Tuesday.

At 8.30pm about a dozen council staff were gathered on the sand waiting for lights to arrive to help them find the black balls in the dark. By this time, the tide was high and many of the balls could have washed back out to sea.

Coogee Beach is accessible by stairs from a seawall and council officers have put traffic barricades in place to block public access to the sand.