It was only when the blindfold was whipped off to reveal a couple of cute baby alligators that the star of shows like the Secret Life of Us and Underbelly II was able to relax.

Johnson also hand-fed Sumatran tigers at the Helensburgh wildlife park as part of a dare to coincide with his visit to the region.

He's completed plenty of dares since he set out across the nation on a unicycle 300 days ago. In that time he's pedalled almost 13,000 kilometres, and raised $1.1 million for breast cancer research.

"I've eaten a live huntsman [spider] in South Australia, I've unicycled for a kilometre underground and along a treetop walk in West Australia and I've bungy jumped on my unicycle in Cairns," he said.

"But it's not every day you get to swim with alligators and feed Sumatran tigers. Once the blindfold was off and I knew they were babies, I was able to enjoy swimming with the alligators.

"And to feed Sumatran tigers, which are such majestic animals, was truly inspiring - although there was one point when one of them growled and started pawing at the fence that got me a little worried."

Johnson has had a lot of fun on his journey, which will end on February 13 at Federation Square in Melbourne. But the Love Your Sister tour has a serious side.

"My sister Connie is battling breast cancer - she's stable at the moment but she is not going to recover as it's spread to her lungs, liver, pelvis, spine and knee," he said.

"It's been a great experience for us to work together on this project - it's provided some meaning in an otherwise meaningless situation.

"And it's been amazing to raise so much money for the Garvan Research Foundation, which are world leaders in biomedical research and serious about finding a cure for breast cancer.

"They developed the drug which is keeping my sister alive."

Johnson's tour has raised awareness of breast cancer.

He is also set to break the Guinness World Record for the longest journey on a unicycle - which currently sits at just over 14,600 kilometres.