"It will be open to some appellants to incorporate the Supreme Court's decision into their appeals," a spokesman for Attorney-General Chris Finlayson said.
Hidden cameras on private property have featured in several prominent drug cases, including Operation Major in 2006, in which police seized 96kg of P and which resulted in life sentences for Wei Feng Pan and Ming Chin Chen.
Police Association president Greg O'Connor said hidden cameras were used to investigate serious crimes, and were involved in most serious drug operations. "There's only one better piece of evidence than a video, and that's an admission."
The Crown could also face appeals if a law was passed, but not retrospective - an aspect of the bill criticised as against the rule of law.
Otago University Law Professor Andrew Geddis said: "Once the courts have said, 'this is the law', then the law ought to apply as it stands. If you meddle with the law to change the outcome of court cases, that is constitutionally highly problematic."
He said without retrospectivity, "the Government could be open to appeals on convictions that relied on covert video footage, if the covert evidence was critical to the conviction".
But improperly obtained evidence could still be admitted under the clause in the Evidence Act that empowers a judge to allow it, weighing up several factors including the seriousness of the crime.
Mr Finlayson said it was not enough to rely on this clause for past cases; a judge could rule footage inadmissible if there were other ways to obtain the evidence, or on expectations of privacy.
To pass the bill, the Government needs three more votes from Labour or Act, who have only said they would support the bill at first reading if it proceeded to a select committee.
Labour leader Phil Goff said the police needed powers to fight serious crime, but objected to ramming the bill through without public scrutiny.
He supported the option proposed by Professor Geddis: passing into law the relevant clauses of the Search and Surveillance Bill, which has passed select committee scrutiny.
LOOKING CLOSER
Option 1: The Video Camera
* Covert video surveillance by law enforcement agencies lawful for all "searches".
* A search is defined as under a search warrant, in circumstances where such a warrant is not needed, or in circumstances where the search would have been legal but for the use of a hidden camera.
* Applies to all searches that take place for one year after the bill comes into force, during which time the Search and Surveillance Bill can be passed.
Option 2: Pass the parts of the Search and Surveillance Bill relevant to video and audio tracking.
* Covert video surveillance with a search warrant legal for investigating offences punishable by seven years' jail or more, and for some firearms offences; legal without a warrant for no more than 48 hours for investigating offences punishable by 14 or more years' jail.
* Covert video surveillance without a warrant from a public place restricted to three hours in any 24-hour period and eight hours in total.
Option 3: The Supreme Court ruling
* Police use of video surveillance involving trespass, even with a search warrant, is illegal.
* Covert filming from public places or from places not involving trespass is legal, subject to the Bill of Rights Act protection against unreasonable search and seizure.
