The Government has no idea what it would cost to have the SIS eavesdrop on New Zealanders and intercept their emails, despite saying cost is the reason for getting the GCSB to do it.
Under the Government Communications Security Bureau Amendment Bill it would be legal for the bureau - the country's foreign intelligence agency - to spy on New Zealanders when assisting other law enforcement agencies.
A public policy researcher says the Intelligence and Security Committee should get the costing done before approving legislation giving the country's electronic spy agency greater powers.
Simon Terry says the alternative is for the domestic spy agency, the SIS, to be given the capability to carry out that surveillance but no analysis has been done of the cost.
Mr Terry has made a supplementary submission to the committee asking it to seek more advice.
Independent MP Peter Dunne also believes some analysis should be done of what it would cost the SIS to carry out the surveillance.
Prime Minister John Key, who chairs the committee, is on holiday in Singapore and not available for comment. Radio New Zealand's political staff say he is unlikely to agree to the request.