The Government will use urgency next week to pass a law ensuring that a recent Supreme Court decision on covert filming will not apply to ongoing investigations.
Illegally obtained evidence resulted in the Crown dropping charges against 13 people arrested in the 2007 police raids over alleged military-style training camps in the Urewera National Park.
The Supreme Court found that using hidden cameras was illegal and police trespassed on land owned by Maori tribe Tuhoe to obtain the evidence.
Prime Minister John Key said on Monday that decision compromises court cases and current investigations.
"Crown Law has advised ministers that almost all use of covert video surveillance by the police is now rendered unlawful."
Mr Key said it is not a situation the Government believes is satisfactory and the law will be changed next week to provide a temporary solution.
"Up to 40 current trials may be affected by this decision and over 50 police operations will be impacted.
"We, therefore, have the immediate and pressing concern that the police are currently left in a position where they are unable to investigate some serious criminal offending."
The Prime Minister said a more permanent law change will be included in search and surveillance legislation to be passed after the general election is held in November.
Mr Key said the Supreme Court decision as it relates to those who brought the appeal would still stand.
The Crown dropped the charges under the Arms Act charges on 12 September after New Zealand's highest court ruled that certain evidence could not be used.
Chief Justice Sian Elias found that filming using hidden cameras for over 10 months was illegal and a serious intrusion on rights. The judge said the fact police knew they were breaching human rights and continued to do so was a significantly exacerbating factor.