An 11-year-old autistic boy has been rescued after four nights alone and lost in a dense Australian forest.

Luke Shambrook was spotted from a police helicopter before noon in the Fraser National Park in Victoria state, three kilometres from his family's campground where he disappeared from on Friday morning, police commander Rick Nugent said.

"We are told that Luke is well, that he is exhausted, he's suffering hypothermia and he's also dehydrated," Mr Nugent told reporters.

Paramedics were assessing him, Mr Nugent said. His parents were overwhelmed, he added.

More than 120 searchers had combed the dense scrubland since Friday morning, with fears mounting that he could have drowned in a nearby lake.

Temperatures fell below 9C during the nights and there had been some rain.

Hopes were raised yesterday when Luke's beanie hat was discovered far from the lake and near where the boy was ultimately found walking.