Adelaide girl Grace Moores was devastated when emergency brain surgery meant she could not go to the Katy Perry concert she had been looking forward to for a whole year.

So she was delighted when the American singing superstar dropped by her house on Tuesday.

Perry heard about eight-year-old Grace's situation after a tweet by her sister Tiana garnered more than 20,000 retweets.

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Grace was diagnosed with a brain tumour in April and had her most recent, unplanned surgery last Wednesday.

Mum Marie Moores said that meant she couldn't go to Perry's Monday concert.

"It was the one thing she had been looking forward to for so long," Ms Moores said.

"Even through all of this, it was like 'I want to get better to see Katy' and the setback last week stopped that a little bit."

Perry's management contacted Tiana through her Twitter account on Monday and then Perry visited the family's Banksia Park home yesterday.

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Grace said it was "amazing" to have the singer of Roar, I Kissed a Girl and Firework pop in.

"She sang three of her songs and she looked at our bedrooms," Grace said.

Perry shared a video of Grace and her family with her 71 million Instagram followers via the social media platform's Stories feature.

She played concerts at the Adelaide Entertainment Centre on Saturday, Monday and Tuesday.

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Tiana was surprised by her quickly her tweet gained retweets considering she only had 50 followers last week.

"I just tried to do a little bit to get Grace to have fun," the 16-year-old said.

"It was amazing to have no status and then to have Katy Perry here."

"She's a very lovely human.

"She just wanted to be there and help make Grace be happy, really."

She next stops at Melbourne for three concerts at Rod Laver Arena on Thursday, Friday and Sunday.