A grandfather has become a millionaire after cashing in a year-old winning Lotto ticket he found while cleaning his car.

The man, from Newcastle in New South Wales, who wished to remained anonymous, bought the ticket in January last year.

But he only discovered it was a winner this month when he checked the ticket online after finding it in the bottom of his Aldi shopping bag.

"I line the bottom of my reusable Aldi shopping bags with old beer cartons to give them some strength," he said.

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"The ticket was hidden in underneath it and I didn't know it was there."

He checked the ticket online, discovering he'd won a division one prize of $1,020,496.

"I'm glad I didn't just get the old bags and take them to the dump," he said.

Cheryl Tuckerman, owner of the Nextra Marketown Newsagency in Newcastle where the ticket was purchased, said she was relieved the 13-month search for the winner was over.

The belated winner plans on using the money to help his children with house repayments and pay for his grandchildren's education.

NSW Lotteries has urged players to check their old tickets.