Thieves have stolen a bicycle from a man who was resting at a caravan park south of Perth three months into a solo ride around Australia for charity.

Chris Anderson, 33, left Frankston in Victoria on May 1 on his bike in an effort to raise money and awareness for Beyond Blue, which helps people living with depression.

After pedalling more than 3,500 kilometres and having battled rough weather to make it across the Nullarbor and the southern half of Western Australia, he pulled into the caravan park in Mandurah on Sunday night.

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However, Mr Anderson got more than he bargained for.

"I heard someone at about three this morning tinkering around outside. I went straight outside, but [the bicycle] was already gone," Mr Anderson said.

"It was my first day in town and my first night here and someone has pinched my bike."

Mr Anderson, who calls himself the 'Lone Aussie Rider', said he was gobsmacked.

"I'm really lost for words, to tell you the truth," he said.

"I just can't believe it actually happened. It's just a low-life act."

Mr Anderson said the distinctive black Avanti bicycle, which features Beyond Blue stickers, cost $1,000 but was donated to him for the charity ride.

The bicycle has a serial number on it and will be traced if it is sold to a pawn broker.

Replacement bike for Lone Aussie Rider

Helpful locals have handed Mr Anderson a new bicycle after hearing about the theft. ( Facebook: Chris Anderson )

Despite the theft, Mr Anderson said he planned to finish the 20,000 kilometre ride around Australia's coast, estimated to take about 12 months to complete.

He has already been given another bicycle from generous locals so he can continue his journey, having raised more than $3,000 towards his target of $20,000.

"There are truly amazing people out there," Mr Anderson said on Facebook after being handed the bike.

It is not the first challenge Mr Anderson has had to overcome since leaving the Mornington Peninsula.

The rider had to have his trailer repaired after its towing arm broke near Manjimup almost two weeks ago, while wet and windy conditions and boredom have also proved testing.

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