The Security Intelligence Service could be able to create an army of informants immune from prosecution, MPs have been told.



Hearings on the Security Intelligence Service Amendment Bill started today, but are being held in private despite efforts by opposition parties to hold them in open.



Council of Trade Unions president Helen Kelly released its submission to media.



It said the bill opened the possibility of the SIS creating an extensive army of informants immune from criminal and civil prosecution.

''We are concerned that this bill sets up structures that will enable random surveillance without sufficient authorisation, at an extreme leading to a situation where there could be a wide network of informants throughout society,'' the submission said.



''While this may not be the intent, we should not set up structures that would allow such outcomes.''



The CTU raised concerns about the wide scope of powers, functions, and work of the SIS.



It was also concerned the bill would have an impact on trade unionists and negatively affect free speech, political activities and privacy.



''Our reluctant acceptance of some of its powers should not be taken as indicating comfort.



''With regard to this bill, we are particularly opposed to extensions and broadening of its powers to recruit informants, and in effect to maintain an army of informants in the workplace and elsewhere in the community. There are grave dangers in the proposals.''



Ms Kelly said the committee should hear submissions in public.



''Hearing submissions in secret has few precedents and stifles public awareness of the dangerous implications of the bill, and of the position of the parties on it,'' she said.



''We certainly have nothing to hide in our submission, which is available to the public, and it is unbelievable that a hearing of submissions on a bill will turn up highly secret evidence. If a submitter wanted to give confidential evidence, he or she could ask for the committee to go into closed session at that point only.''



Prime Minister John Key earlier defended the secret hearings saying the established rule was that the committee held hearings in private unless its members unanimously agreed otherwise. He argued it was not in the public interest for any holes in security to be aired publicly.



The move to have hearings in open was defeated by the National, Maori Party and ACT MPs on it.



The bill updates and modernises the laws covering the SIS, and deals with the warrants framework, electronic tracking devices, computer surveillance and technology like mobile phones and cyber identities.



It doesn't change the threshold for warrants that are needed for those purposes but it makes explicit provision for them in law.



Mr Key is the minister in charge of the SIS and is the automatic chairman of the committee.



He appoints two members, and when the committee was formed after the last election he named Maori Party co-leader Tariana Turia and ACT leader Rodney Hide.



Mr Goff, an automatic member as Leader of the Opposition, nominated Green Party co-leader Russel Norman.