Gavin Kleinhans set to travel around Australia on a postie bike

But Gavin Kleinhans, also known as the "Postie Geezer", is a man who revels in taking the road most difficult.

The 65-year-old retiree and his Honda CT100 will leave Sydney's Bondi on April 25 to raise funds for Medecins Sans Frontieres/Doctors Without Borders - and test himself in the process.

"I'm one of these people who likes to be challenged," says Gavin, who was born in South Africa and as a young man was conscripted into the military and "fought in all sorts of wars on and off".

"So I was taken outside of my comfort zone at a very young age. And those experiences - or that feeling - of being outside your comfort zone and coming through it stays with you," he says.

"And that's what I attribute this ride to. Every so often I get a bee in my bonnet."

The father of two is going anti-clockwise from Sydney to avoid the worst of the heat. Along the way he will catch up with friends, while his partner, Coleen, will fly to meet him at stops along the way.

But other than that, he will be on his own. He has chosen not to have a back-up team.

"That's kind of the whole notion behind it; to do this as a solo ride, to be restricted in terms of what I can do."

Make no mistake, though. Gavin is a very experienced rider - his last big trip was from London to Mongolia and his regular bike is a Ducati 950 Multistrada - and he has thoroughly prepared his little machine.

"I've fitted the bike up with extra fuel capacity. I put an additional tank on I'll carry a tiny little five-litre jerry can."

Gavin's bike has only a 110cc engine, so to minimise encounters with heavier vehicles - think road trains! - he will ride on back roads most of the time.

These include Gibb River Road in WA, usually the domain of well-equipped 4WD adventure drivers, and a stint west of Cairns where he will be travelling for 400 kilometres on dirt tracks through the mountain ranges with no towns or communities along the way.

To keep in touch, he will carry a GPS navigator and satellite communicator with the ability to exchange text messages and emails.

The device also provides detailed weather updates and offers emergency response services.

"The advantage is that once I leave, people can follow me in almost real time. Every two minutes, it will send off a tracking signal saying this is where I am.

"It also has a shared map that people can click on to locate me.

"The whole idea is that no one has to worry about me doing this. The worst thing is just having people asking 'Where's Gav, I haven't heard from him'."

Clearly, the main determinant of distance is fuel range.

"You need to be able to obviously traverse some long distances. But there are other things like hills and headwinds. They slow you down dramatically.

"I'm working on between 300 and 350 kilometres a day. If I do that, I'll be quite chuffed.

"I've calculated that if I do 80 per cent of that and ride every day, it'll take me about 55 days to complete 15,500 kilometres. But I won't be riding every day."

Gavin happily admits his postie bike is no speed machine.

"Level on the flat without wringing the thing's neck, you can you can certainly you can rev it at full taps. But I don't want to do that - I've got 15,000 kilometres to go.

"So I'll set it about 70-75 kilometres an hour - that would be the cruising speed on the flat. Down hills, you can get up to a bit more.

"Going up hills, of course, the revs just drop away so quickly. And sometimes I'm down to first gear and literally crawling at that stage!"

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He acknowledges that it will not be a comfortable trip, "by any stretch".

But some things might surprise.

Gavin jokingly says on his website that "I don't bend easily anymore and 'I ache in places where I used to play' (to quote Leonard Cohen)."

But asked about the potential for rattled bones, he said: "I'll tell you something. The seat on that postie bike is the most comfortable motorcycle seat I've ever sat on.

"I'm six foot tall, so I thought I'd be really cramped up on this thing. But not at all. The riding position is quite comfortable.

"But I have supplemented it with a what's called an Airhawk, which is a seat that you just slightly inflate.

"It provides a bit more cushioning - not really necessary, but I've done it anyway.

"I did a test ride six weeks ago - 1000 k's in three days. And I was all right, surprisingly. I thought I wouldn't be able to move after sitting on it for a few hours.

"Obviously, I'm not going to be riding every day. I want to enjoy the ride. I want to take in some sites, spend some time in places - for example, Coleen will meet me in Maroochydore. So she's going to see some parts of the country as well."

Gavin is up front about what he is in for.

"I'm not so sure about the postie bike. I'm not sure that, one, it can make it, or two, I can make it.

"But it's those that kind of notion that appeals to me; the fact that there's a lot more unknown moving parts to this.

"I'm coming up to 66 in a few weeks and you come to a point where you say, 'Will I ever be able to this, if I don't do it now?'

"And I've always been one of those people that when an opportunity pops up, I never want to turn away from it. Because you never now, it may never come again."

"The last thing I want to do in life, like most of us, is to live with any regrets at all; to think, 'God, I could have done that, if only I had the courage to do. I could have done it then but I can't do it any more'."

He has fitted fully adjustable rear shocks and is carrying special luggage panniers for his camping gear and other essentials - including a drone!

Gavin's fundraising target for Medecins Sans Frontieres is a modest $5000, of which he has raised $2600 already. To chip in, go to fundraise.msf.org.au/fundraisers/postiegeezer.

To see his YouTube channel showing off his bike, click HERE.