The boy told Mr Flannery his four-year-old sister was in trouble in the water.

Mr Flannery said he ran down to the boat ramp to see the girl was stuck under the jetty.

"I saw her under the water and then she came up and grabbed onto the pontoon.

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"When she popped up she was screaming and pretty distressed."

He returned to the unit to grab a life ring and a tow rope.

"When I went back down I couldn't see her but then she she popped up in the middle."

Kaitlyn Riley, 16, who was nearby jumped in the water to help.

"She [Kaitlyn] came down and jumped in the water straightaway with no no hesitation," Mr Flannery said. "She got underneath and grabbed hold of the girl and pulled her out from there.

"If it wasn't for her who knows what would have happened.

"It shows there are good people in the community."

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Mr Flannery said it appeared the four-year-old girl and her older two brothers were swimming by themselves.

"Supervise your children when they are in and around the water at all times and always swim with a buddy so if something happens they can alert someone and stay calm," he said.

"It was quite scary knowing the situation we were faced with.

"I have been on plenty of search and rescues, but haven't had to do anything like that before."

Mr Flannery called triple-zero when he was altered to the situation and the girl was taken to hospital as a precaution.

A Tasmania Police spokesman said the girl had sustained minor injuries and there were no suspicious circumstances.

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