NZ First leader Winston Peters is claiming the Prime Minister's staff were told about Kim Dotcom's application to buy his mansion in July last year - but John Key says he wasn't told.

Key has maintained for months that the first he knew of the German millionaire was on January 19 - when he was briefed on a police raid that was due to take place the next day.

Peters has produced Key's answer to a written parliamentary question to back up his claim.

In the answer, Key confirmed one of his staff was advised by former Cabinet minister Simon Power's office that he was declining an Overseas Investment Office application from Dotcom to buy his $30m Coatesville home.

But Key said: ''This information was not conveyed to me as it was routine.

Peters said is is ''hard to fathom'' how Key can claim he didn't need to know about Dotcom's application to buy the property.

''Why would Mr Power relay the information to the Prime Minister's office if it wasn't for the consumption of the Prime Minister?''

- © Fairfax NZ News