Following more instances of Employers coming second over the enforcement of 90 Day Trial Periods here are some resources that will help you with a correct implementation of the trial period.
The Employment Relations Authority are very strict regarding the process by which the 90 Day Trial Period gets implemented with the Employment Agreement. Any failure in process can invalidate the use and subsequent termination of employment under the supposed protection of the 90 Day Trial Period.
www.employers.co.nz/90day-trial-period-tools.aspx