Two young siblings who were involved in a crucial bone marrow transplant have been granted a "once-in-a-lifetime" audience with King Charles III and Queen Camilla, who are in Australia for their first royal visit since the king's coronation.

Adelaide boy Ky, who was diagnosed with a serious blood disorder three years ago, and sister Charlotte, who became his donor, were among those who gathered to meet the royal couple on the tarmac at Sydney Airport on Friday night.

Ky told ABC News Breakfast his enthusiasm for all things regal stemmed from the royal family toys his dad used to bring back from his trips to the UK.

As Queen Camilla passed along the assembled line, the 12-year-old gave her a bouquet of waratahs.

"She liked the flowers a lot," he said.

"I was slightly nervous, but I was also excited."

Charlotte, who is in year 12, said all her school friends were "shocked" and "excited" by the news that she'd be meeting the royals — an opportunity that was organised by the Make-A-Wish Foundation. 

"If you tell anyone you're meeting the king and queen, it's going to be hard to believe, but they were all excited and I've been keeping them updated," she said.

"It was absolutely incredible — what a wish. 

"It was a once-in-a-lifetime opportunity and [Ky] took it in his stride, talking to the king and queen — so yeah, it was absolutely amazing."

Ky was aged just nine when in 2021 he was diagnosed with severe aplastic anaemia, after his mother noticed a large bruise on his side.

His family was living in the United States at the time, but flew back to Adelaide.

They were told he would not survive without a bone marrow transplant, the Make-A-Wish Foundation said.

All three of his siblings were tested, but Charlotte turned to be the match he needed, and she donated 1.6 litres of marrow.

"I was so relieved to find out that I was a donor, because there's such a slim chance — 25 per cent — that he was going to find a donor," she said.

"It was more relief than anything through that process, knowing we wouldn't have to go search for other donors on the registry."

The foundation said Ky's wish to meet the royals was also inspired by a video clip of Queen Elizabeth II having tea with Paddington Bear during the Platinum Jubilee celebrations.

His mother Loretta said that while it had been a small scramble to spruce up and get to Sydney, the whole occasion was "really lovely".

"We only got notified a week ago that this was actually going ahead," Loretta told ABC News Breakfast.

"It's been very quick, but Ky got his suit and everyone else got new outfits.

"We got to stand on the tarmac as well, a little way back, and watch Ky and Charlotte.

"There was a big heavy downpour just before it all happened, but then it cleared up and it was beautiful."