Teen drowns at lake near Perth after twin brother raises alarm

A teenage boy has drowned at a popular lake in the Perth Hills.

His twin brother raised the alarm after it's believed the 17-year-old became trapped under a pontoon.

The tragedy unfolded at Lake Leschenaultia in Chidlow, roughly 40 kilometres east of the CBD, about 2pm (5pm AEDT) today.

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Locals armed with scuba gear joined the search, with up to 30 people in the water trying desperately to find him.

"We dove so far, it was so dark, we got the boats out, moved the pontoon all until the police and ambulance came, we did everything," said Emerald McKelvie, one of the first to respond. 

But the murky water made it almost impossible.

"Everyone was shaken up, everyone was in tears in the end, because we couldn't find him. We didn't have the –," McKelvie said. 

"It was eight metres, man, I'm gonna say, definitely, and then it's just darkness."

Police divers found the teen 45 minutes later.

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"We tried in that first 10 minutes. We tried to keep him back," McKelvie said.

"His mum was screaming, his brother was screaming."

They were seen hugging each other in grief after the drowning.

The lake was immediately closed after the death and swimmers were told to avoid the area.

Police were treating the teen's drowning as a terrible accident.

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Early indications suggested he became trapped under the lake's large pontoon. 

It's not known if the family are local, or visiting for the school holidays.

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