The Goverment has the numbers to progress a bill next week to enable police use of video surveillance while investigating crimes.
But the Act Party announced this afternoon they would support the bill at its first reading on the condition that it would be subject to 36-hour select committee scrutiny.
"If the Act caucus was happy with the bill as it came out of select committee, we would support it," parliamentary leader John Boscawen said.
The bill would suspend the effect of the Supreme Court decision earlier this month into the Urewera case.
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With United Future and Act, the Government no longer needs Labour Party support for the bill to pass its first reading.
The bill would effectively mean police, under a search warrant, can still use hidden cameras on private property to gather evidence - a practice the Supreme Court said was illegal, but which police believed was legal on the common law presumption that it was not unlawful.
"The Act Party is not prepared to give the Government a blank cheque," Mr Boscawen said of the select committee condition.
He said the party did not think the bill was needed to bring serious criminals to justice.
"We don't believe that is correct."
But he was concerned that police covert video operations had been suspended and police cases were potentially jeopardised.