Four-year-old Austin Vearing has cerebral palsy. One of the ways the condition affects him is muscle tightening.

Austin is set to travel to the USA in June for surgery, which parents Kat and Julian Vearing hope will help him to walk independently and live with less pain.

The family is seeking to raise $97,000 to cover the costs of the medical procedures, as well as rehabilitative equipment.

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To celebrate Austin's fourth birthday, which happened to be on Christmas Day, Mr and Mrs Vearing planned a four-kilometre virtual walk or run.

They invited people to get involved in their own communities, at any time on Christmas Eve, and to donate to a GoFundMe page in Austin's name.

Several community members took the initiative on board, with Mrs Vearing's gym organising a walk around Crusoe Reservoir on Thursday morning.

That's where Austin's birthday celebrations kicked off, with him riding around the reservoir on his bike.

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A visit to the Melbourne Star Observation Wheel was among the family's plans for the rest of the day, as well as a monster-themed birthday cake Mrs Vearing had made at Austin's request.

Mr Vearing wished to say a big thank-you to everyone who had supported and shared the virtual event.

The GoFundMe tally climbed above $20,500 on the morning before Austin's birthday.

To donate, visit: gofundme.com/f/austins-wish-to-walk