Iron Boy - a brave Sydney boy with cystic fibrosis - has earned a gallantry award and been appointed an honorary Avenger by Robert Downey Jr himself, after saving the city in a mission organised by the Make-A-Wish Foundation.

Domenic donned his superhero suit after accepting the Make-A-Wish mission to save reporter Hope Joy, who had been taken hostage on Clark Island by the evil villain Ultron.

Domenic as Iron Boy.

The pint-sized hero arrived at Clark Island at lunchtime with his sidekick and brother Joseph, who played the character of 'War-Machine'.

The pair courageously rescued Ms Joy from the villain’s clutches, and untied her from a tree.

Iron Boy then had a final showdown with his arch nemesis at Sydney Opera House, and defeated the villain in front of crowds of cheering onlookers.

He was presented with the Commissioner’s Award for Gallantry for his efforts by NSW Police Commissioner, Andrew Scipione, who commended Domenic’s courage during the battle and participation in the event.

“Not only has Domenic battled villains and saved his city, he’s stared down his illness and won the day,” he said.

“It can’t be easy dealing with an illness like his and we feel privileged to be able to work with Make-A-Wish Australia to ensure his wish to be a famous superhero came true.

“It was also my great honour to present Domenic, or should I say ‘Iron Boy’, with a special acknowledgement, a Commissioner’s Award for Gallantry.”

Robert Downey Jr, the actor who plays Iron Man, then appointed the brave superhero an honorary Avenger in a special video message.

The Hollywood actor also tweeted "Go get 'em, Domenic!" to the boy, adding that he "sent a very special boy on a top secret mission today".

Aussie actor Chris Hemsworth, who graces the big screen as Iron Man’s friend and fellow superhero Thor, sent out a simple "Go Dom!” on Twitter.

Mr Scipione had earlier briefed Domenic on the mission.

"Iron Boy, we need your help,” Mr Scipione told Domenic via video message, in the "top secret" operations room.

“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," he said.

Iron Boy makes his way to Clark Island, where Ultron and his henchmen have gathered.

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

According to Make-A-Wish, when Domenic was asked what super power he would possess above all else, he answered “to heal”.

In a video posted to the charity group’s website, Iron Boy’s mother gave a glimpse into the kind of superhero he is, saying he was “always more interested in helping people rather than playing with toys”.

“He was a very caring child, more interested in helping children and other people rather than playing with toys.

“He always wanted to know what was going on around him in the world, how he could help others.

“I guess the final straw was when he just sat up at the dinner table and said ‘I am Iron Boy’ and we pretty much had to go with it from then.”

Make-A-Wish and NSW Police earlier stirred up excitement across Twitter in the lead-up to the unveiling of Sydney’s newest super hero.

A series of fictional news reports by reporter Hope Joy were filed on Twitter, which asked the public for help in identifying an ‘anonymous do-gooder’, who thwarted a ‘doughnut robbery’, leaving behind only a note signed “You’re welcome! Iron Boy”.

There have also been ‘reports’ of Iron Boy flying across Sydney’s skies.

“Police received a flood of reports from passengers travelling on a local airline saying they saw Iron Boy flying across the skies of Sydney Harbour,” Hope Joy reported, promising to bring more developments, before she was kidnapped.