'Iron Boy' Saves Sydney With Downey Jr Support

A nine-year-old boy with cystic fibrosis has seen his wish come true after he became superhero 'Iron Boy' and fought crime in Sydney.

Domenic Pace has idolised Marvel's Iron Man for years and dreamt of becoming his own, smaller version of Robert Downey Jr's character.

On Thursday the Make-A-Wish Foundation fulfilled his dream and gave him a day to remember in Australia's biggest city.

In scenes reminiscent of 2013's Batkid - a five-year-old boy recovering from cancer who became the diminutive version of Batman in San Francisco - crowds turned out to watch Pace save the day on the harbour in a spectacle that took almost a year to organise.

"Sent a very special boy on a top secret mission today. Go get 'em, Domenic! #IronBoyAU @MakeAWishAust," Downey Jr tweeted.

New South Wales Police Commissioner Andrew Scipione told Pace, who was playing in a Sydney park when he was flown by helicopter to police headquarters for the surprise mission, that authorities needed his help.

"Make-A-Wish reporter Hope Joy has been kidnapped. Officers have confirmed that Clark Island has been overtaken by Ultron's henchmen and Hope Joy is being held captive," Commissioner Scipione said in a video recording.

"If I could make one wish, it would be for you to help us, Iron Boy."

Pace was fitted with a specially made Iron Boy suit and his 12-year-old brother Joseph played the role of Iron Man sidekick Colonel James Rhodes as he told reporters he was "ready to get down to business".

The schoolboy was then whisked off in a boat to Clark Island in Sydney Harbour, where he freed the fictional reporter before engaging in a fight with villainous robot Ultron on the steps of the Opera House.

"Go Dom!," Australian star Liam Hemsworth, the younger brother of Chris Hemsworth who plays Thor, tweeted as #IronBoyAU became the top-trending hashtag on Twitter in Australia.