This is the requirement under the tenancies laws of South Australia - taken from the OCBA site. As you can see there is NO % or $ amount stipulated as a cap on rent increases. There is no necessity for a landlord to have made any improvement to the property. There is however provision for the amount of notice given to a tenant before an increase is made. To be able to haggle down an increase is quite unusual. Increases are generally made at the commencement of the next agreement period so you should be notified before the current term is up, and then you have the option of accepting it or looking for another place to rent.
The landlord may increase the rent under the following circumstances:
where the terms of the tenancy agreement allow it (i.e., for fixed term agreements,
provision for rent increases must be written into the lease);
where allowed, rent can only be increased after six months into the tenancy or six months
from the last rent increase;
the tenant must be given 60 days' written notice of the landlord’s intention to increase the