This owner need to take an action at the CTTT under section 149 (below) to get an order to refund the excess payments she has made, using the Unit Entitlements that were registered with the strata plan as her guide, and to make future levies compliant with the Act.
What should happen is that the Owners Corp should refund money to those who have overpaid and charge an excess to those who have underpaid. The CTTT may place a time limit on how far back you can go with your claim.
The first step is to apply for mediation at Fair Trading (this form) and if that fails, make an application for an order at the CTTT using this form.
Realistically, the other owners don’t have a leg to stand on if there is evidence that they have been charging levies calculated by an other means than the unit entitlements.
This owner will have to provide evidence of what the unit entitlements are (or should be) and what they have paid in levies over the years.
This is what section 149 of the Act says:
149Order for variation of contributions levied or manner of payment of contributions
(1) An Adjudicator may make either or both of the following orders if the Adjudicator considers that any amount levied or proposed to be levied by way of contributions is inadequate or excessive or that the manner of payment of contributions is unreasonable:
(a) an order for payment of contributions of a different amount,
(b) an order for payment of contributions in a different manner.
(2) If an Adjudicator considers that the consent of an owner referred to in section 77 has been unreasonably refused, the Adjudicator may make an order for payment of a different amount of one or more contributions levied or proposed to be levied by the owners corporation.
(3) If a contribution which is the subject of an order under this section has been wholly or partly paid:
(a) an order to pay more has effect as if the owners corporation had decided to levy a contribution equal to the difference, and
(b) an order to pay less imposes a duty on the owners corporation to refund the difference.
(4) An application for an order under this section may be made only by the lessor of a leasehold strata scheme, an owners corporation, an owner or by a mortgagee in possession (whether in person or not).