Pipi Quinlan made the winning 20,000 New Zealand dollar ($12,300) bid on the Kobelco digger with a few mouse clicks at the auction site TradeMe while her parents slept, the Rodney Times newspaper reported in northern New Zealand.
"The first I knew about it was when I came down and opened up the computer," said Pipi's mother, Sarah Quinlan.
"I saw an e-mail from TradeMe saying I had won an auction and another e-mail from the seller saying something like `I think you'll love this digger,'" she was quoted as saying in the paper.
Quinlan said she had made auction bids on several toy sets and assumed she had bought a toy digger.
"It wasn't until I went back and reread the e-mails that I saw $20,000
- and got the shock of my life."
She immediately called the auction site and the seller to explain what happened.
TradeMe reimbursed the seller's costs for the auction and the digger was relisted.