A West Auckland man has discovered the Lotto ticket he had been using as a bookmark for almost two months is worth $20.1 million.

The man, who has chosen to remain anonymous, said the winning ticket had been sitting inside a book since July. He only found it when he picked the book back up, following a break from reading.

"I put the ticket straight into my wallet ready to check the next time I went to the shop," he said.

He knew Blockhouse Bay Postshop & Stationers had sold a winning ticket, but "didn't put two and two together". The ticket could easily have stayed on the bookshelf for months, he said.

After scanning the ticket at a self-check kiosk, he initially believed the "major prize" alert meant he had won "likely a few thousand dollars".

"It still didn't cross my mind that it would be $20m."

The Lotto operator "was more excited than me" about the win, he said.

"I think I was in shock."

He said he would share the news of his lottery win with close family in the near future, as he decides what to do next.

"I'm going to take things slow and really think about what to do with this money.

"I do want to go on an overseas trip, but that's the only plan so far."

In August, Lotto asked players to check their tickets after the prize went unclaimed for several weeks.

"We're so pleased the lucky winner found his ticket and checked it before we started the next stage of our search," Lotto NZ head of corporate communications Will Hine said.

"I can only imagine what it must have felt like to realise that his bookmark was worth $20.1m."

There's still one more unclaimed prize in Auckland - a $333,333 winning ticket was sold at Four Square St Heliers for the 27 June draw.

"We'd love to get this prize to the rightful owner, so if you might have bought a ticket in St Heliers in Auckland a few months ago, please have a look for it - and maybe have a scan of your bookshelf," Hine said.