Got this from the Tenancy website
For most tenancies, the rent cannot be increased within 180 days of the beginning of the tenancy, nor within 180 days of the last rent increase. The landlord must give the tenant at least 60 days’ written notice to increase the rent in addition to a period allowed to correctly serve the notice. There is no limit as to how much a landlord may increase the rent by, as long as the rent is not increased excessively above market rate.
Rent can only be increased for fixed-term tenancies where provision for this is written into the tenancy agreement.
If the rent is excessive, the tenant may apply to the Tenancy Tribunal for a market rent assessment. If the Tribunal finds that the rent is too high, it may order a reduction in the rent. Market rent applications can only be filed for fixed-term tenancies within 3 months of the beginning of the tenancy or 3 months of the date of the last rent review.
Hope this helps...