When two smiling strangers knock on your door, you usually expect them to be selling something - not to tell you that you're now a multimillionaire.

But that's exactly what happened to one Christchurch man.

After a Big Wednesday $22.6 million prize went unclaimed for more than three weeks, Lotto New Zealand staff took matters into their own hands.

Using ticket information stored in the gaming systems and CCTV footage from Pak 'N' Save Riccarton, where the winning ticket was bought, they found the man they were looking for.

But even hearing he had won the largest-ever lottery prize in the South Island from two Lotto officials, the winner still had to be convinced to check his ticket.

Lotto NZ chief operating officer Chris Lyman, who delivered the life-changing news, said the man was convinced somebody else had won the prize so had not bothered checking his ticket.

"They said 'I think you're our Big Wednesday winner'. I just said 'nah, it's never me'. I knew somebody already won that prize - a guy at work told me,'' the winner said.

Despite the huge win, the man is determined the money won't change his life dramatically - except now he won't have to work until he's 65. He said he loved Christchurch and would not want to live anywhere else.''But maybe now I can build the house of my dreams here,'' the man said.

"When someone wins a prize this big, we usually hear from the winner within days, if not hours,'' Lyman said.

"This guy was sitting on a winning ticket worth $22.6m and - as incredible as it sounds - he may have never checked it," he said.

Lotto NZ corporate communications manager Emilia Mazur could not release details about the man but said he was married with children and had played Lotto for many years.

''I think $700 was his biggest win before this,'' she said. Mazur and Lyman spent about 20 minutes with the man before they left to ''give him some time to process'' and check his ticket.

''Telling someone they've won that amount of money . . . well, it's a pretty amazing feeling,'' she said.

She said there had been a lot of talk in Christchurch about the unclaimed prize and who the ticket holder could be.

For ''privacy reasons'' she could not say how staff found the man's address after identifying him on security footage.

The money was now safely in the winner's bank account. ''The first thing he wants to do is help his family out as much as possible and then I think an overseas trip is on the cards.''

Sam Stacey, from Riccarton Pak 'N' Save's Lotto store said staff were told this morning the man had claimed the prize.

She said the unclaimed win had been a talking point in the supermarket for weeks. ''It's crazy . . . he'd been sitting on that ticket for quite a while.''

Our [manager] always lets us know about big prizes . . . and whether or not they've been claimed and it was, 'No, not yet' until this morning.''

Stacey said no staff knew who the winner was and said the fact Lotto was looking for the man had been ''kept pretty quiet''.

Riccarton Mall management would not say whether or not it played a role in helping to track down the winner.

Ursula Cheer, from the University of Canterbury's school of law, said a complaint to the privacy commission could only occur if someone had "suffered harm, such as humiliation or distress".

"But I would say in this instance, the person was quite happy to be tracked down."

She said the fine print on Lotto tickets would likely state what information could be held but she was "quite interested" in how the security footage was accessed.

"Maybe there's an arrangement in place between Lotto and the supermarket . . . but it does make you feel a bit uncomfortable."