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"Ultimately it reflects badly on our Parliament. (I) can't comment on the sentencing cause that hasn't taken place yet but it's certainly a sad day," Mr Key told reporters in Cairns where he is to attend the Pacific Island Forum.
He said it would be "unjustified" for this case to be taken as a reflection of the whole Parliament, he said.
"I think it's an isolated case. I think Mr Field's been dealt with now by the judiciary and ultimately has to face whatever sentence they decide is appropriate. I think New Zealanders should see it on that basis."
He said it showed investigations into Mr Field ordered by the former Labour government were not looking for a solution.
"What it shows you is when you are facing a very serious issue you actually have to have a proper investigation and I don't think the terms of reference were ever set in a way where the government of the day was look for a conclusive outcome. Actually they wanted to park the situation for political convenience."