LATEST: Party secretary sticks to Key line on Sabin

Prime Minister John Key denies he knew about an assault complaint against Mike Sabin before last year's election.

NBR understands the PM was first made aware of the assault complaint in April last year, months before the September 20 ballot - and that the National Party knew before the 2011 election.

The Northland MP resigned on Friday, citing personal reasons.

Mr Key said this morning he knew about Mr Sabin's resignation "a day or two prior".

Asked, "Can we absolutely say you knew nothing about this before the election?" the PM replied "No ... I was aware of the personal family matters for about the last week of Parliament last year [Dec 8 - 12]. So that's the timeframe."

NBR asked the PM's office to clarify whether he knew about the assault complaint before the election.

Chief press secretary Sia Aston replied, "To be clear, the Prime Minister wasn’t aware of any personal family matters prior to the election."

NBR asked if it were possible to view the assault complaint separately from the personal family matters, or if they were inter-related.

Ms Aston replied, "You can take my answer in the broadest context."

On Breakfast, Mr Key praised Mr Sabin, saying he was a very good local MP who would well have gone on to become a minister.

The PM said advice he'd received from the Electoral Commission pointed to a mid-to-late March byelection in Northland.

Mr Sabin held Northland at the 2014 election with a 9300 majority. He was first elected to the seat in 2011 (with an 11,362 majority), when he replaced the retiring John Carter. After the 2014 election, he was promoted to chair the Law & Order Committee.

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RAW DATA: John Key talks to Breakfast's Rawdon Christie

Christie: Mike Sabin's gone .... the question is how long you've known about it.

Key: In terms of his personal family matters, he's put out a statement saying he left Parliament for personal family reasons. I was aware that A) He was going to resign a day or two prior to that and I was aware of the personal family matters for about the last week of Parliament last year [Dec 8 - 12]. So that's the timeframe.

Christie: So way past the election? Because there are rumours you did know something about this before the election, which would make it awkward for you, given the fact you endorsed his position as a star MP in the National Party.

Key: Absolutely I endorsed him and he's been a very good local MP up North. Frankly I think he could well have become a minister. He had a lot of potential. He has very strong policy ideas, both for economic development for the north and in relation to drugs; he came into Parliament as an expert in that area.

Christie: We can absolutely say you knew nothing about this before the election?

Key: No.

Christie: The byelection - any more details?

Key: The Electoral Commission advice I've had - I just need to fine-tune one point, but I'd say mid to late March. There's a couple of potential opportunities from the 14th to the 28th, but it'll be around that time.