Wu said he showed a staff member the photo so they could match the price.
But Wu was told the guarantee didn't cover promotions.
"I thought that was a bit dodgy," he said.
So Wu went home and checked the fine print, but the only exclusions were "trade and special quotes, stock liquidations and commercial quantities".
"They should really honour their promise," Wu told the Herald.
He ended up buying the washing machine from Noel Leeming but contacted the Herald with his frustrations because he felt if companies made a guarantee, they "should be prepared to meet that promise otherwise that's just false advertisement".
However, when the Herald contacted Mitre 10 its chief executive Neil Cowie said Wu was correct, a staff member made a mistake and the price guarantee should have been honoured.
Cowie said the person who made the error was "mortified" and passed on their sincere apologies.
Wu was given vouchers to the tune of $60 - the 15 per cent discount he should have got.
He said he'll give the same amount to the Auckland City Mission as it wasn't really about the money for him.