He said the MPs who had left or were leaving Labour were also mainly right wing - a nod to the likes of Shane Jones, Clayton Cosgrove and Phil Goff.
Jones was poached by National to take up an ambassadorial appointment in 2014. That is due to end in May next year and Jones is rumoured to be considering a re-entry to politics with NZ First.
Asked if National was in talks with any other Labour MPs, Key turned coy saying "oh, we always talk to people." Pressed further he said a cheery "see ya!" and ended the standup.
Key said it was up to the National Party members in Mana to decide whether to select Leggett, but he was welcome in the party.
"He's made it clear he wants to stand for National because he thinks Labour has gone too far left. Probably you would argue the way they were polling, that's probably what the rest of the country thinks as well and if he wants to put up his hand, we are happy to accept him."
That was a reference to a One News Colmar Brunton Poll this week which had Labour up slightly at 28 per cent and a Roy Morgan poll yesterday which put Labour at 23 per cent while the Greens went up three points to 14.5 per cent. In both, National was around 50 per cent.
Labour is likely to reject the Roy Morgan poll results - in September, Labour leader Andrew Little had dismissed Labour's 26 per cent result in a Colmar Brunton poll as 'bogus' and released internal polling to try to refute it.
Key himself said he did not put too much faith in the Roy Morgan results which tended to bounce around. "You always want to take that poll with a grain of salt. Even though it's working for me at the moment, I think we'll give it a bit of a wide berth."
However, he said in general National's vote was remaining consistently high in polls at about 48 - 50 per cent while Labour's was moving around.
Nick Leggett, a former Labour member spurned for being "right wing" by Labour leader Andrew Little announced yesterday that he had signed up with National, saying there was too much to put at risk by changing the Government.
Leggett confirmed he had joined the National Party this week and intended to seek selection for National in the Mana electorate, pitting him against Labour's Kris Faafoi.
He pointed to the uncertainty prompted by the election of Donald Trump, the Brexit vote in the United Kingdom and the rise of extremists on the left and the right.
Listen: Nick Leggett talks to Newstalk ZB's Larry Williams about his switch to National.
"I have observed the work of the Government and of John Key as Prime Minister. I've seen more and more that New Zealand's place in the world - in quite an unstable world - is becoming more critically important."
"I think those sorts of issues coupled with really competent government, economic management and looking for new ways to provide social services - the National Party have done a really good job and there's too much to put at risk."
Leggett was Porirua Mayor from 2010 to 2016 and a long-standing Labour Party member until he contested the Wellington mayoralty which pitched him against Labour's candidate Justin Lester.
In August, Little banned MP Stuart Nash from appearing in a debate which featured Leggett, saying Leggett was "a right-wing candidate" who took donations from right wingers and was part of a group trying to undermine the Labour Party.
Leggett denied he was acting out of revenge. He had ended his Labour party membership at the beginning of the year "but I had been Labour in name only for a good many years."
"I hoped the party would change and broaden and open itself to being the centre party it once was, but I realise with the MoU with the Greens and things like the Chinese names saga, it's not the party it once was and it's not the party for me.
"I'm clearly a centrist and a liberal and National is the broad church that allows that."
He took aim at Labour's stance against the Trans Pacific Partnership and policy to rein in immigration as factors that weighed in his decision, saying trade and being tolerant to diversity were important.
Earlier this week Prime Minister John Key said National would welcome Leggett if he wanted to join.
Little told Newshub on Saturday that he was not worried by the prospect Leggett would move to National, saying he was "not true Labour."
"Labour people, passionate Labour in their heart, they stick with Labour, they campaign on Labour issues, and for the Labour Party. Nick's not one of those people."
It is the latest in ongoing friction between Labour's hierarchy and its right wing rump, including over a bid to set up a centrist ginger group within the party by former candidates Josie Pagani and Phil Quin.
The party's moderate MPs such as Shane Jones, Clayton Cosgrove and Phil Goff are leaving and some saw the decision to sign an agreement with the Green Party as a further move to the left.
Little also recently raised eyebrows by saying he did not intend to pitch to the centre of politics.
Leggett said it would be an "uphill battle" to win the Mana electorate from Faafoi, whom he described as a popular MP. Education Minister Hekia Parata has stood for National in Mana in the past but is leaving Parliament in 2017.
Faafoi won the seat by almost 8000 votes in 2014 but Leggett has some profile in the electorate from his years as mayor.
