Starship paediatrician Dr Greg Williams is already a superhero to countless sick Kiwi kids and their families.

And now he is about to reveal another special power, plus a different side of himself, around the country.

Williams is cycling the length of New Zealand to raise funds for the Starship Foundation. Along the way he'll transform into children's book crime-fighter Captain Underpants – dressed in "tighty whities" and red cape.

"I'm a paediatrician. I'm fundraising for sick children. So something child-focused seemed appropriate," Williams said.


"[And] my theory was that people would rally to a grown man making himself look ridiculous for the sake of a good cause."

Dr Greg Williams' greatest superpower on display was a
Dr Greg Williams' greatest superpower on display was a "willingness to put pride on the line". Photo / Supplied

He raised around $7000 for the Foundation when he ran the 2015 Auckland Half Marathon dressed as the book character.

The popular Captain Underpants children's novel series revolves around two primary-school age pranksters who hypnotise their principal into thinking he is a superhero who fights crime while wearing only underwear and a cape. The principal – aka Captain Underpants – ends up acquiring superpowers.

An animated movie based on the novels was released last year and a TV series is scheduled for this year. But not everyone knew about the character when Williams, 52, ran the half marathon.

"People on the sideline as I was running through Auckland would yell out, 'Go Superman' and I'd have to yell out, 'No, not Superman – Captain Underpants!'.

"I'm not really the right build for it either (at 188cm), cause he's short and spherical, and I'm basically the opposite of that. I've had to take quite a bit of artistic licence in adopting this character."

Williams heard about the character through his two sons. "They had all the books."

He had "sped-read a few of them", and found them "great … the right amount of irreverence".

Williams, who started the Tour Aotearoa ride at Cape Reinga on Wednesday, was "not planning on riding 3000km in cape and Y-fronts". "I don't like to think about the consequences involving a lot of chafing and sunburn."

But he would be transforming into Captain Underpants for special appearances. His greatest superpower on display was a "willingness to put pride on the line".

He will carry his own gear and sleep in a bivvy bag along a range of cycle trails, tracks and lanes to Bluff. He plans to make the journey in around 20 days.

He will be accompanied on the ride by Starship paediatric oncologist Dr Mark Winstanley, from Auckland to Wellington.

Williams, who has been at the Auckland-based national children's hospital for 10 years and heads its general paediatrics ward, said the strength of his young patients would get him through the toughest parts of the epic ride.

"I come across a lot of children and young people whose resilience and inner strength is just constantly surprising.

"So when I think of all the kids who I've cared for, who have risen against adversity, whether it's in their upbringing or the kind of conditions and diseases that they're coping [with], that strength carries me a long way."

Williams knows the importance of the Starship Foundation, which has raised $135 million since its inception more than 25 years ago to help the hospital save and improve the lives of generations of Kiwi youngsters.

"Our ward [was] rebuilt just a year ago and it was a multi-million-dollar project, and the Starship Foundation provided half of that.

"Whether it's equipment or research or support for families - the support they provide reaches far and wide."