The 15-year-old has spent the past four years fighting the insidious disease. She was first diagnosed at the age of 11.

Shay's mother Jules Keath said the third diagnosis came as a complete surprise.

"Shay relapsed in April, which was very unexpected because we were still half in treatment from last year," she said.

"Because the cancer came back the doctors had to reassess her treatment and have decided she needs a bone marrow transplant."

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Shay's father will donate bone marrow for the transplant, scheduled for June 21 at Melbourne's Royal Children's Hospital.

Ms Keath said to date, immune therapy had eliminated the cancer but the bone marrow transplant was necessary to ensure it didn't come back.

Shay and her mother have been living in Leukaemia Foundation accommodation since April 17, away from their brother and son Jayd, 12, and Ms Keath's fiancé.

"We've been home once but we're stuck here until we move home in October because Shay has to be close to Melbourne," Ms Keath said.

"We don't get to see Jayd or my fiancé unless they come down on the weekend, and because of the coronavirus lockdowns we can't really go out."

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Ms Keath has had to put her photography business on hold to be there for her daughter.

"It's hard to get food here and then my partner is feeding my son in Bendigo, plus we need money for fuel. There are some out-of-pocket medical costs as well," she said.

To help cover the expenses, Ms Keath's family and friends have started a GoFundMe page to try to raise $30,000.

In 10 days, the online fundraiser raised $7477 from 56 donors.

Ms Keath said she was thankful for the support for her daughter and their situation.

"Shay is just a very positive young lady with a beautiful smile and she's always thinking about others," she said.

"Even though deep down I'm sure she's thinking about what's going on, she takes it in her stride.

"She misses her friends but she's a very determined young lady, so I'm sure she'll get through.

"Every time we got through this we get more resilient; you just do it."

Ms Keath said people could donate to Shay's fundraiser at gofundme.com/f/shays-3rd-cancer-battle.

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