MPs have been warned new anti-terror legislation, as it stands, would allow spies to go on fishing expeditions for information.
A select committee was sitting through a second day of urgent hearings on a bill which aimed to crack down on terrorist fighters.
The bill would allow the Security Intelligence Service to spy on suspects for up to 48 hours without a warrant.
Privacy Commissioner John Edwards said if agents found nothing of interest, they could then stop, the only consequence being they would have to report they had done so.
"They could have a camera in your bedroom, they could be listening to all your telephone communications, they could be intercepting and viewing your emails and your internet traffic and that could happen for 48 hours just on the say-so of a public servant."
He said the SIS could spy on someone without a warrant and then stop within 48 hours, the only consequence being it would have to report that it had done so.
Former director of the Government Communications Security Bureau Sir Bruce Ferguson said he was particularly worried about giving agencies the power to spy on people for up to 48 hours without a warrant.
"I'm not convinced that you cannot get a warrant within 48 hours if it is that concerning then you can actually have people who are authorised assign warrants on standby if you are worried about the [cricket] World Cup coming up."
He said extreme caution was needed in giving agencies and politicians that kind of power.
The committee will report back to Parliament on Tuesday.