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It is the first in a series of meetings which Mr Key hopes will allow him to put together a government within a week.
He will hold talks with United Future leader Peter Dunne later this afternoon and Maori Party leaders Tariana Turia and Pita Sharples tomorrow morning.
National, with 59 seats, only needs ACT's five seats to govern, but Mr Key has said he is keen to form as broad-based a government as possible.
Mr Hide entered negotiations today with no bottom lines, but he spelt out several priorities.
However he said his party's support was not in doubt.
"We've always said we will support a John Key-led National government and we will," he told reporters.
"It's important for the country that we have a stable and secure government to give us a sense of direction."
Top of the policy wish list was capping government spending and scrapping the emissions trading scheme (ETS) - something Mr Key has already committed to keeping in an amended form.
Mr Hide refused to accept that and said dumping ETS, which put too many costs on farmers and businesses, would form part of negotiations.
ACT also wanted to push its "three strikes" policy for violent offenders.
He said the country needed a change of direction.
"We're not going to throw our toys out of the sandpit but we certainly think that we face challenging times and we campaigned on a change of direction, not just a change of government, and we are looking for that change of direction."
Mr Key has already ruled out former Labour finance minister and ACT No 3 Roger Douglas from a ministerial role, but Mr Hide said the exact number and nature of ministerial roles given to ACT was one of the things up for negotiation.
He said it was too early to say if ACT would seek ministerial positions inside Cabinet in a formal coalition or outside Cabinet in a looser support arrangement.
Initially Mr Key was left hanging as he emerged from his offices for a pre-arranged handshake for media with Mr Hide only to be told he had not arrived yet.
Mr Hide then made Mr Key wait inside the glass doors of the National offices for about two minutes while he spoke to media.
The pair then shook hands and entered talks.
ACT deputy leader Heather Roy and National deputy leader Bill English were also present.