A man has drowned while trying to rescue two children believed to have been caught in a rip at a Western Australian beach.

Two children were believed to be struggling in the water at Native Dog Beach near Bremer Bay, south-east of Perth, about 2.10pm (5.10pm AEDT) on Monday.

Four adults nearby jumped in to help but also ran into difficulty.

During the attempt to save the two children, a man in his 40s became unresponsive.

Members of the public helped him to shore and gave him first aid.

When emergency services arrived, the man was taken to Bremer Bay Nursing Post but could not be revived.

Police confirmed the children and remaining adults were able to return to shore safely.

A report will now be prepared for the coroner. 

It was the first of two drownings in the state this week.

A 17-year-old boy became trapped under a pontoon at Lake Leschenaultia in Chidlow about 2pm yesterday.

A teenage boy has drowned at Lake Leschenaultia in Chidlow. (9News)

His twin brother raised the alarm but, despite efforts from dozens of locals, he could not be found in the murky water.

“Everyone was shaken up, everyone was in tears in the end, because we couldn’t find him,” Emerald McKelvie, one of the first to respond, told 9News.

“His mum was screaming, his brother was screaming.”

Police divers recovered his body about 45 minutes later.