Opposition parties are calling for a broad inquiry, but Mr Key said it will instead be confined to the conduct of Ms Collins and her relationship with Adam Feeley, the former head of the SFO.
Ms Collins is accused of trying to undermine Mr Feeley when she was Minister of Police. An email from Slater in 2011 said the minister was "gunning for" Feeley. She has denied any wrongdoing.
Other private emails released over the weekend appear to show right-wing bloggers were being paid to run a smear campaign against the Financial Markets Authority and SFO, overseen by Carrick Graham, the then-PR handler for former Hanover boss Mark Hotchin.
The terms of reference will be released today or tomorrow, but Mr Key outlined the nature of the inquiry during his post-Cabinet press conference yesterday.
Opposition parties are asking for a Commission of Inquiry or a Royal Commission to look into wider issues raised in Nicky Hager's Dirty Politics book, including the possible misuse of information by ministerial offices and government departments, and the accessing of Labour's computer system by Jason Ede, who was then working in the Prime Minister's office.
But Mr Key rejected a broader inquiry.
"I just don't think that's warranted. The Opposition would want to make those claims because they simply want to use this as a way to make it bigger than it is, or to smear the Government.
"This sort of quaint little notion that there's a lot more going on, or that the left of politics don't talk to bloggers, don't do things, all the rest of it, it's a lovely little notion that might be running around in David Cunliffe's head but it's [not true]."
He noted the release of SIS information to Slater was being investigated, and invited the public to make police complaints if they had any criminal suspicions about the SFO investigation or other issues.
He said it will be independent, likely chaired by a retired judge or a QC, and include all the powers in the Inquiries Act, passed last year.
He would not say if the terms of reference would allow the inquiry to expand during its work.
"It won't be a Royal Commission."
It will also not look into Ms Collins' conduct when she gave the details of a public servant to Slater, who then abused the public servant on his website, prompting death threats.
"It was dealt with when I looked at it and said it was unwise," Mr Key said.
"It's not in dispute that she gave those details. I could spend time on that, but I don't think it's really worth it."
Under the Inquiries Act, public or government inquiries have the power to summon witnesses and order evidence including documents or other information.
Refusal could see a conviction and a $10,000 fine.
Mr Key said he did not condone what seemed to be a smear campaign against the SFO, but said there was no evidence to suggest that it affected the SFO's operations.
Meanwhile, he confirmed that his office's staff will give evidence to the Inspector-General of Intelligence and Security, who is investigating the release of security information to Slater.
- by Derek Cheng, NZ Herald
