A charity auction for a Palmerston North woman's piece of Prince Charles' birthday cake ended up raising twice what the treat sold for.

Liz King was one of 64 guests at a birthday celebration for Prince Charles at Government House last week.

This week she auctioned her piece of cake from the party for her niece Sophie King.

The cake was sold last night to a bidder from Dannevirke for $450. Donations from individuals and businesspeople who saw the auction gave Miss King a further $457 in donations to take the tally raised past $900.

Sophie, 6, lives in Masterton and has endured 14 operations over 24 months after a tumour was discovered on her brain stem when she was 2.

Miss King lives in Palmerston North and put the slice of cake online to raise money for a Gold Coast holiday for Sophie and her family.

"It's just fantastic," she said.

"I thought it'd raise a couple of hundred dollars to put towards the trip to the Gold Coast."

Miss King said eating the cake herself would have been "a waste" and she had instead wanted to do something good with it.

She was grateful to those who had made contributions.

"Thank you so much, it's going to make a real difference," Miss King said. "Hopefully next year [Sophie] will be able to get to the Gold Coast. She's been through a lot and it's fantastic how people have supported her."

The holiday gave the 6-year-old something to work towards as her condition improved.

The family had been unable to take long holidays due to Sophie's condition, Miss King said.

"They've been away for one or two days, but before now they've been scared to go too far away from a hospital that knows Sophie's history."

The piece of cake is the second piece from the prince's cake to have been sold on Trade Me.

Wellington resident Sally Jamison put a piece of the cake on the site last week, after her son-in-law attended the birthday bash at Government House.

That piece sold for $555.

Anyone wanting to make a donation to Sophie can do so through the bank account 12-3143-0241390-50.

- © Fairfax NZ News