"In most cases where an employee has successfully challenged the use a trial period by their employer, the courts have focused on technical slip-ups made by employers," she said.
"These meant that the employer could not rely on the trial period and the employee was able to raise a personal grievance about their dismissal."
The courts have strictly applied the requirement that the trial period applies only to new employees.
In one case, an employee did not sign her contract until her second day of work. The court ruled she was not a new employee and the trial period was unenforceable.
In another case, a worker accepted a verbal job offer that made no mention of the trial period. The court ruled he was an existing employee by the time he signed a contract just one hour after starting work, and the trial period could not be enforced.
Ms Townsend said employees were entitled to know their employment would be subject to a trial period before accepting job offers.
There was also a common misconception that trial periods effectively extinguished all of an employee's legal rights within the first 90 days of employment.
"When it appears the employee's performance or attitude is not meeting requirements, there is still an obligation to address this during the trial period," she said.
A boss who took no steps to deal with performance issues during the trial period could face claims of unjustified disadvantage and breach of good faith, she said.
Ms Townsend said the law imposed stringent requirements for the use of the trial period and the courts had made it clear they would be strictly applied.
"A 90-day trial period must be included in a written employment agreement, applies only to new employees, can be no longer than 90 days and must start on the day the employee begins work."
