"It was only after I'd filled in the prize claim form and went to show the wife that I found out how much we'd actually won.
"A bunch of our friends and family were around and I was pretty chuffed so I strolled into the lounge and told everyone we'd just won $18,000 with Lotto.
"Everyone was excited, but then my wife had a proper look at it and she said 'ah honey, no - we've won $18 million'.
"It was pretty unreal. Everyone thought we were pulling their legs and we'd set up an elaborate prank. We had to phone the Lotto NZ help desk to confirm that we really had won that much money."
The couple, who are expecting their first child, are understandably "over the moon" with the win and are still coming to terms with what it means for their future.
"We're trying to keep level headed about it and not go crazy. We'll pay off the mortgage, we'll help out family and give some money to charity," the husband said.
"But we're most excited about the life we can now provide for our baby and the opportunities this creates."
The winner's wife, who screamed when she heard the money was going to be in their bank account overnight, has been enjoying sharing the news with close friends and family.
"We phoned some friends yesterday to tell them that we had some news, but because we're pregnant everyone's first guess was that we were having twins," she laughed.
"They certainly weren't expecting to hear that we'd just won Powerball."
The couple bought their ticket on MyLotto.co.nz and the prize is made up of $18 million from Powerball First Division and $250,000 from Lotto First Division.
It was the first Powerball win this year.
