"Anyway we're working it out," he said before before heading into National's weekly caucus meeting.
Australia's mining boom has helped to keep it out of recession during the global financial crisis and Mr Key was likely referring to that.
But the mention of coalmines in New Zealand at present is synonymous with tragedy with the Pike River mining disaster being relived at the hearing of the Royal Commission of Inquiry in Greymouth.
However, Grey District Mayor Tony Kokshoorn said Greymouth people were not offended by Mr Key's comments.
He said more coalmines would boost New Zealand's economy.
"If he wants to up the bet, he can call for half a dozen more coalmines. Then we'll take it."
In comments on the nzherald.co.nz facebook page, some readers labeled his wager insensitive.
Trish Coyle said: "He is a frat boy - not a leader. His comment is insensitive to say the least, but what can you expect from the man behind 'Party Central."
However, most came to the Mr Key's defence, saying his comment was a harmless joke.
Rebecca Taylor said: "Give the guy a break, He has a tough job trying to run the country and keep everyone happy. So what he shared a bit of humour its not like he committed some heinous crime."
Sherry Lee said: "Oh lighten up people. Just because he is a politician, does not mean he has to be sombre. He is a great man and has a great sense of humour".
Kirsty-Funshine Harvey said: "I think we are lucky to have a PM with a sense of humor."
Others thought the joke was simply guilty of falling flat.
Leith Gibbons compared it to Mr Key's 2009 appearance on the Letterman Show in which he read out a jokey top 10 reasons to visit NZ.
"It's not that it was offensive, it's just that it was really, very lame. Not nearly as lame as the time he was on Letterman, that was a whole new level of embarrassment on an international scale."
- HERALD ONLINE STAFF