The Solicitor-General has been summonsed by Parliament and asked to reveal the advice on the legality of the lockdown.
Director general of health, Ashley Bloomfield, and police chief Andrew Coster have also been summonsed by the Epidemic Response Committee.
Chairman of the committee and Opposition leader Simon Bridges said the move was unprecedented and Parliament had never summonsed the Solicitor-General before.
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"The people of New Zealand have given up their freedoms for this lockdown. We all deserve to know what the legal basis was for that," Bridges said.
"New Zealanders should be proud of the efforts they've made during this lockdown but they also deserve to know whether the lockdown was legal."
Bridges said National had been asking for the legal advice for the past five weeks after "serious concerns" were raised by academics, lawyers and the Law Society.
The move was unprecedented, he said.
When Bridges was an MP in 2013, he was asked to release legal advice when WorkSafe dropped the case against Pike River boss Peter Whittall.
At the time he told RNZ: "In terms of making legally privileged documents public, you'll know the Crown never does that."
All legal advice on the legality of lockdown and ongoing restrictions has been sought.
Act leader David Seymour said there was "overwhelming" public interest in releasing the advice.
"The people deserve to know on what basis they have had their civil liberties restricted. The advice is particularly relevant for business owners who have been shut down by the police and citizens who have been turned away at checkpoints or arrested."
The legal basis of the lockdown has been a point of contention at the committee.
Attorney-General David Parker, a Labour MP who acts as chief law officer of the Crown, appeared before the committee on April 16 and said he was confident the Government followed the law.
Parker said the law was "abundantly clear" that governments "can for reasons of public health restrict people's liberties".
But the advice wouldn't be released because of legal privilege, he said.
Leaked emails from Deputy Commissioner Mike Clement to district commanders three days after lockdown revealed Crown Law opinion warned police had little or no power to enforce the lockdown.
Two weeks later, Bloomfield used the outdated Health Act to issue specific regulations.
Parliament's Regulation Review Committee has written to Bloomfield saying there are many questions over the lawfulness of the orders he's issued, in particular isolating and quarantining people who aren't sick and closing businesses.
Court of Appeal president Stephen Kos said last Friday the rules and how they are being applied has raised many questions.
Prime Minister Jacinda Ardern has previously said the Attorney-General was satisfied all powers were appropriately used and police had not been acting illegally during lockdown.
She was confident that the lockdown was legal.