An inquiry into the release of a sensitive security document to Whale Oil blogger Cameron Slater within days of requesting it will include access to computer systems, documents and phone records.
In an unusual step, Inspector-General of Intelligence and Security Cheryl Gwyn provided an update about her inquiry this afternoon, in recognition of public interest.
She is looking into whether anything inappropriate took place when the Security Intelligence Service provided Slater with response to an Official Information Act request just days after it was requested, and which was not given to media outlets that had asked also made OIA requests at the time.
"The scale of the inquiry is significant - it involves access to a variety of IT systems, documents, phone records and so forth," Ms Gwyn said.
"I then need to analyse that information, discard what is irrelevant, and then take into account the evidence from those summoned."
The released information showed that the then-SIS director Warren Tucker had briefed then-Opposition leader Phil Goff on an incident that he had previously denied being told about.
Nicky Hager's book Dirty Politics raised questions about whether Slater was tipped off or given preferential treatment as part of a politically-motivated attack.
"It is important that the inquiry is comprehensive, fair and independent," Ms Gwyn said.
"I will not be cutting any corners."
The inquiry will not be completed until well after the election.
It will begin nine days before the election, and individuals will give evidence in private over more than a week.
Slater, Mr Goff and several members of the Prime Minister's office have been summoned. John Key has not been, although he has said he would be happy to appear.
Jason Ede, who used to work in the Prime Minister's office, is expected to give evidence as well. Dirty Politics alleges that Mr Ede helped Slater with an OIA request in relation to the restructuring plans at the Ministry of Foreign Affairs and Trade in 2012.
Ms Gwyn said all witnesses will appear under oath.
"I am adopting this practice in respect of all those who I interview for the purposes of this inquiry."
Anyone who fails to comply with a summons without reasonable excuse can be fined up to $5000.