Iron Boy, aka Domenic Pace, has defeated Ultron in a vicious battle on Sydney's Opera House steps after the superhero earlier rescued a reporter from his evil clutches.

The nine-year-old had earlier made a trip by police speed boat to Clark Island to save Make-A-Wish Foundation reporter Hope Joy after she was kidnapped by Ultron's evil henchmen.

Who is Iron Boy? Nine-year-old Domenic is battling cystic fibrosis

Nine-year-old Domenic is battling cystic fibrosis His dream is to be Iron Boy for a day and rescue someone and help save the city

His dream is to be Iron Boy for a day and rescue someone and help save the city Today, the police chief called Iron Boy out on a special mission

In a tough battle witnessed by hundreds of cheering bystanders, Iron Boy not only defeated his arch-nemesis Ultron, but also a group of henchmen, on the Opera House steps.

The superhero was then congratulated by NSW police chief Andrew Scipione who called him "Sydney's true hero" before being presented with a medal.

"It was also my great honour to present Domenic, or should I say Iron Boy, with a special acknowledgment, a Commissioner's Award for Gallantry," Commissioner Scipione said.

"Not only has Domenic battled villains and saved his city, he's stared down his illness and won the day."

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Domenic, who has cystic fibrosis, was granted his dream to be Iron Boy for a day by the Make-A-Wish Foundation.

He said his ideal super power would be to heal people.

The south-west Sydney schoolboy started his day in a park where he was recruited via helicopter to go on a secret mission in a joint project between the foundation and NSW Police.

This morning Commissioner Scipione himself called on the nine-year-old to help rescue Ms Joy and stop Ultron.

"Iron Boy we need your help. Make-A-Wish reporter Hope Joy has been kidnapped," Commissioner Scipione said in a video addressed to Domenic.

"Officers have confirmed that Clark Island has been taken over by Ultron's henchmen and that Hope Joy has been held captive.

"If I have one wish it is for you to help us Iron Boy."

Iron Boy saves Make-A-Wish reporter from Ultron's henchmen. ( Supplied Image: Make-A-Wish-Foundation )

Domenic watched the message from NSW Police Headquarters then nodded as he was asked by Assistant Commissioner Mal Lanyon if he was prepared to save the day as Iron Boy.

It did not take long for Iron Boy to spring into action, first travelling to police headquarters in Parramatta by a police helicopter to be briefed on his special mission and to put on his armoured suit.

He then travelled to Clark Island by police speed boat from the Parramatta Warf to start his mission.

The young superhero grinned broadly as he was handed the high-tech crime fighting ensemble.

He told the police and media he was "ready to get down to business".

In an interview on 2GB, he confirmed that the chopper ride had been "awesome".

Domenic is presented with his Iron Boy suit at NSW Police HQ. ( ABC News )

"We saw a couple of fields, the Three Sisters... and Warragamba Dam," he said.

"And we went through a bunch of cloud."

When asked if he was going to save Sydney, the nine-year-old replied, "Yep".

Assistant Commissioner Lanyon said the force was thrilled to be a part of the plan hatched by Make-A-Wish.

"It's a great opportunity to do something for a little boy and obviously give back to the community," Assistant Commissioner Lanyon told 2GB.

His mother said Domenic had always had a fascination with Iron Man.

"From a young age he would only answer to Tony [Tony Stark is Iron Man]," she said.

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

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

She said one day he stood up at the dinner table and said "I am Iron Boy" and the family "pretty much had to go with it from then".

Hollywood's Robert Downey Jr, whose alter ego is Tony Stark/Iron Man, has tweeted his support saying he's sent a "very special boy on a top secret mission today".

"Go get 'em Domenic!" he said.