Mr Key this week stood by Mr Banks, saying he had not broken the law.
He reiterated that on TVNZ's Q+A programme this morning.
"I accept his word, I accept the law is very ambiguous, and I accept that the Labour Party is using this as a politically motivated attempt to get to the Government.''
Mr Key said he did not need to read the police documents.
"I haven't read that police report and I'm not going to because I don't need to ... It's not my job to do a forensic analysis. What I can tell you is, the law doesn't work.''
The Government is seeking a law change that would put a $1500 cap on anonymous donations to local government campaigns.
Mr Key said Dotcom's statements had to be seen in the context of the law.
"The fact that someone tells you they may make you a donation, doesn't mean under the old electoral law that you know that they have.''
Mr Banks has previously denied asking for the donations.
Asked if lying was a sackable offence for ministers, Mr Key said: "The test is whether they enjoy my confidence, and if a minister tells me, 'This is my position and this is what I've done', I accept their word in good faith unless it's proven otherwise.