A judicial review that will consider the legality of the level 3 and 4 lockdowns has been given significant legal grunt.

The Criminal Bar Association and the Auckland District Law Society are joining forces with Wellington lawyer Andrew Borrowdale, who is mounting the case.

The New Zealand Law Society is also party to the action, but their role as intervenor is neutral and independent of the parties to the case.

Questions have been raised as to whether director-general of health Ashley Bloomfield had the legal authority under the Public Health Act to effectively shut the country down and to order people to stay at home unless they had good reason for being out.

Crown Law advice - leaked during the lockdown to Newstalk ZB - suggested Bloomfield did not have the required authority, which would throw into question all the arrests made during it.

The same advice was used by the former Deputy Police Commissioner Mike Clement to warn his district commanders about the thin ice they were on when it came to making orders in the name of Covid-19.

Attorney General David Parker has insisted the leaked Crown Law advice was a draft copy, even though it had the same serial number as the advice used by Clement to warn his officers.

Borrowdale, a former legal draftsman at Parliament, is seeking a judicial review of the lockdown handling. The case is scheduled to be heard in the High Court at Wellington next month and it's understood in a relatively rare move there will be a full bench of judges hearing the matter.

The court last week heard from the Auckland District Law Society and the Criminal Bar Association, which wanted to become involved in the case.

A judge also wanted to give the Law Society the opportunity to become involved and gave them until today to declare its intention, which the society has now done.

It's expected the intervention will give specialist grunt to the case being mounted by Borrowdale.

Law professor Andrew Geddis said the case "is very much a serious one in terms of raising issues that the legal world think are both genuinely uncertain and important."

Geddis says Borrowdale was raising concerns in a proper fashion, unlike another private case taken by two men which failed, after they appealed the High Court finding.

President of the Court of Appeal, Justice Stephen Kos, said at the time that extraordinarily complex questions needed answers. He referred to an article academics Geddis and Claudia Geiringer wrote and a report of Parliament's regulations review committee looking at government powers in emergencies, which he said was "hardly approving".

Geddis said the intervention of the three legal bodies showed they were keen on the legality being properly tested in court.

He said if the judicial review finds the lockdown rules were legally invalid it would have implications for anyone charged with breaching the rules.

* Clarification: The NZ Law Society has applied for leave to intervene in these proceedings. The society's role as intervenor is neutral and independent of thee parties to the case. The society has a statutory mandate to "assist and promote, for the purpose of upholding the rule of law and facilitating the administration of justice in New Zealand, the reform of the law".