"Labour won't get there, their numbers aren't strong enough.
"It's a very unstable mix of four or five parties that really don't agree with each other."
Mr Key discussed the National Government's time in office, emphasising its track record during the global economic crisis.
"It's been quite a journey over the last six years," he said.
"When we came in, it didn't feel like a lot of clear air."
He said there was "nothing sexy" about getting the economy going, with no "silver bullet" to revive things overnight.
Mr Key said New Zealand was now the 5th fastest growing economy in the OECD, with unemployment at 5.6 per cent.
On migration, he said New Zealanders were working it out for themselves.
"They are voting with their feet, staying in New Zealand."
He said that in the past six years, there had been very little discretionary spending.
Labour was promising an $18 billion spend, the Greens $10 billion, he said.
"They are literally promising to spend every budget for four years."
Mr Key said he predicted a low voter turnout, owing to the "dirty politics".
"It turns voters off."