Canberra, Sep 2 (IANS): A major search by sea, land and air is underway for a teenager who was swept away by a wave in South Australia (SA).

Mehdi Habibi, 17, was visiting Granite Island - a small island off the coast of the town of Victor Harbor approximately 70 kilometres south of the state capital of Adelaide - with family on Sunday when he was swept from rocks by a wave at approximately 3:15 p.m. local time, Xinhua news agency reported.

His disappearance triggered a major search and rescue operation that resumed on Monday morning after being suspended on Sunday night due to challenging conditions.

Speaking at a press conference on Monday, Sahil Habibi - the missing boy's younger brother - said he was with his brother on the southern side of the island when he was hit from behind by a giant wave.

"He slipped, he dipped under the water," he said.

Sahil said he saw his brother struggling in the water before vanishing.

Their aunt, Fahema Habibi, said the family was on the island - a popular spot for penguin spotting - for a picnic.

She said that Mehdi arrived in Australia seven months ago from Pakistan and was attending school in Adelaide.

"Everyone is worried, everyone is not sleeping," she said.

SA Police and PolAir were on Monday searching waters around the island using boats and several helicopters while police officers and volunteers from the State Emergency Service were conducting foot searches of the area.

"The search is ongoing this afternoon - conditions out there are not favourable - however, we will continue searching until we locate him," SA Police Inspector Gene Vaninetti said.

"The family is extremely devastated. They are on the island under the guidance of police, but naturally, they are devastated."

Granite Island has been closed to the public while the search is ongoing.