Mr Ambrose said the recording was accidental but Mr Key said the recording was illegal and laid a complaint with police.
Police eventually decided not to lay charges but Assistant Police Commissioner Malcolm Burgess said investigators had concluded the recording was more likely than not to have been deliberate.
Last year Mr Ambrose said he was considering defamation action against Mr Key for saying that he broke the law.
He claimed to have lost "tens of thousands of dollars" worth of work as a result of the controversy.
"I became completely disillusioned with the people running the country. And that's coming from someone who's been a National voter for 18 years", he told reporters in May last year.