A Perth woman won lotto twice thanks to a complete mistake on her part.

The elderly woman accidentally played her own numbers twice, when she meant to buy one of the tickets using her partner's numbers.

The Monday and Wednesday Lotto in Australia offers $1 million to up to four players. Because of her mistake, the woman ended up winning $2 million.

"I used my Playslip for this week's Monday and Wednesday Lotto and when I realised I hadn't used my partner's Playslip, I felt so guilty that I couldn't tell him what I'd done," she said in a Lotterywest statement.

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"I think he's forgiven me now, though."

The winner says she often fills out Playslips to choose her own numbers.

She already knows what she'll do with the money.

"The win will mean a bit more travelling, we'll replace the car and it gives us an opportunity to be kind and generous to others," she said.

Lotterywest spokesperson Hermione Coleman called it a "lovely story and a fantastic win for Western Australia".