Mr Key said the law change would apply retrospectively so filmed evidence already collected could be used. But it would not apply to the Urewera case, for which he said the Supreme Court judgment should stand.
The Crown Law Office had advised that almost all covert filming by police was now unlawful, and the decision could affect up to 40 pending trials and more than 50 police operations.
Police had halted all covert filming until the law is changed.
Mr Key said the police and the Government had believed police were legally entitled to use covert measures to gather film evidence. But the Supreme Court ruling showed there was a need to clarify the law.
The change would be a short-term measure until the Search and Surveillance Bill was passed in the next term of government.
That bill has provisions - including the need for warrants - for secret filming on private property in serious cases, including arms offences.
Lawyer and Labour candidate Michael Bott - who initially represented one of the Urewera accused - said the change legalised actions the police had known were unlawful.
"The police regard this Government as a canteen that never shuts. They can just roll up to the counter and the Government will hand over what they ask for."
Police Association head Greg O'Connor said it was an "essential" change because the Supreme Court ruling could derail significant work by the police.
"The fact the Government has acted quickly and is prepared to use retrospective legislation shows how serious the implications of the ruling are."
Mr Key said he could not yet say whether National had enough support for the legislation to pass.
The Government had briefed other parties yesterday.
"If we can't get the numbers, that would mean there would be some very, very serious criminals who would not be brought to justice."
The Urewera four who still face charges - Emily Felicity Bailey, Tame Wairere Iti, Te Rangikaiwhiria Kemara and Urs Peter Signer - will face trial by jury in February.
