The letter was triggered by an ongoing campaign of harassment against the University of Canterbury professor - apparently linked to her work researching China's foreign policy - that has included burglaries of her home and office and suspicions her car had been sabotaged.
Those co-signing the letter include Amnesty International New Zealand executive director Tony Blackett, investigative journalist Nicky Hager, AUT policy observatory director Julienne Molineaux, intelligence analyst Paul Buchanan and nearly two dozen academics from institutions ranging from Otago to Auckland.
The call for action has also drawn support from both the Green and ACT parties, marking the first time the affair has attracted the interest of Parliament.
Ardern said in February when news of the Brady burglaries broke that suggestions the crimes were a result of Brady's work were concerning and, if substantiated, she would be "taking stock and taking action".
In comments since she has declined to engage on the issue - including stalling for six months on a request to discuss the matter with the Herald in an interview - and has repeatedly said the case was a matter for police.
To date police have made only one brief comment on the affair, saying in September while it was a "complex case" they had "positive lines of inquiry" and Interpol were involved.
It is understood a secretive branch of police, the national security investigations team typically employed to handle terrorism and espionage cases, is working the case.
Both Green Party MP Golriz Ghahraman and ACT MP David Seymour said they agreed with the thrust of the open letter and added their weight to calls for the government to take action.
"Academic freedoms are paramount to the health of our democracy," Ghahraman said.
"Any evidence of threats or intimidation aimed at undermining academic independence must be taken seriously by the government."
Seymour said the nine-month silence from government on the issue was concerning.
"It's been far too long for an issue as important as academic freedom, and ultimately New Zealand's freedom and sovereignty," he said.
"It makes you wonder if Winston and Jacinda are living on their knees because they're too scared to die on their feet - and this position is not acceptable to rest of the country."
TIMELINE
2017
•September - Academic Anne-Marie Brady publishes her 'Magic Weapons' paper, detailing China's campaign of influence in New Zealand, to international acclaim.
•December - Brady's campus office broken into.
2018
•February - Brady tells an Australian parliamentary committee hearing her home had recently been subjected to a suspicious burglary, and the following day her office is again broken into. Prime minister expresses concern and police begin an investigation.
•September - The Herald reports the police investigation is still ongoing and Interpol and NZSIS involvement suggests official suspicions are directed offshore.
•November - Brady's mechanic reports her car has been tampered with, widening the police investigation. Twenty-nine academics and civil society figures co-sign open letter urging government action on the case.
