In the past at least one person had to be aware the conversation was being recorded for a recording to be admissible.
In its decision the ERA acknowledged the decision was not usual: "Recording others secretly when the person recording is not participating in a meeting or conversation with them is generally not admissible."
Firman also recorded a meeting between herself and salon owner Kelvyn Glading where she was given a letter outlining a disciplinary process and suspension was discussed.
This recording was made without Glading's knowledge but was also admissible.
Some lawyers say the case sets a worrying precedent and caution bosses to cement strict policies to protect privacy and water-cooler conversations.
Lawyers from Simpson Grierson said accepting a recording where no party was aware they were being recorded set a worrying precedent.
"The concerning part was where she recorded a conversation with other staff members as she had left her mobile phone in the room when they were talking," Judge said.
"These days everyone has a cellphone which means recordings can be made very easily."
Judge warned employers of the need to protect themselves and their employees by having clear policies around filming and recording in the workplace.
Employers could pre-empt the risk of secret recordings during disciplinary processes by recording the meetings themselves and asking whether anyone wished to record the meeting.
"Employers and employees need to be careful and precise with what they say ... and they need to be aware other employees may be recording them," Judge said
"Our advice is to put clear policies in place and to make clear making recordings in the workplace is not acceptable behaviour."
But other employment specialists say the workplace is protected by New Zealand law.
Peter Moore from Phil Butler Law said the case was interesting because recording a conversation without being party to it is illegal under the Crimes Act.
"Anyone who records a conversation in this way opens themselves up to prosecution," Moore said.
"Just because it is deemed to be admissable by the ERA doesn't mean the person in question is precluded from being prosectued criminally."
"It could be a Pyrrhic victory."
Moore said the case didn't necessarily set a precedent because decisions made by one member of the ERA did not have to be followed by the next.
"Quite often they do because they like to be consistant but they take all aspects of a case into consideration and are not bound by previous decisions," he said.
A spokesperson for the Employment Relations Authority spokesperson said evidence was accessed on a case by case basis.
"There is no necessary flow on from one case to another."
The case Firman v Insyn Ltd (trading as Synergy Hair) has been heard by the ERA but the decision has not been released.
When contacted by the Weekend Herald yesterday Firman declined to comment but she confirmed she is now employed at a new salon in Christchurch.
Salon owner Kelvyn Glading did not respond to an email from the Weekend Herald.