Depending on how efficient your strata managers are, the first thing you might need to do is to apply for an interim orders instructing the owners corp not to proceed with the work.
You would do this using THIS FORM. The application for interim orders does not require mediation but it must be related to an application for orders which does.
So, at the same time you could apply for mediation, then orders, under section 24, revoking the decision of the general meeting, on the grounds that it a) contravened the terms of a by-law already in place and b) gave exclusive use of an area of common property without appropriate common property rights by-laws being passed.
Once that has been resolved, you could apply for mediation, then orders under Section 232, due to a failure of the owners corporation to enforce a legitimate by-law. You probably have to do this after the Section 24 matter has been resolved, due to the conditions laid out in Section 232.3 (below)
In both cases your mediation application would be on THIS FORM and your subsequent NCAT application would be on THIS FORM.
Here are the relevant sections of the Act and how they may apply in your case.
24 Order invalidating resolution of owners corporation
(1) The Tribunal may, on application by an owner or first mortgagee of a lot in a strata scheme, make an order invalidating any resolution of … the persons present at a meeting of the owners corporation if the Tribunal considers that the provisions of this Act or the regulations have not been complied with in relation to the meeting.
Section 135 of the Act requires all owners to comply with by-laws. The decision of the meeting is one that allows owners not to do so and is therefore probably in breach of the Act.
(3) The Tribunal may refuse to make an order under this section only if it considers: (a) that the failure to comply with the provisions of this Act or the regulations, or of the Strata Schemes Development Act 2015, did not adversely affect any person …
The granting of exclusive use of a section of common property and the payment of the costs of concreting are both at a cost to other owners and the actual use of this new parking area may also adversely affect other owners (if that’s the case).
231 Interim orders
(1) If an applicant for an order by the Tribunal under this Act requests the making of an interim order and the Tribunal is satisfied on reasonable grounds that urgent considerations justify the making of the order, the Tribunal may:
(a) make an interim order in the form of any order that could otherwise be made by the Tribunal …
This is worth pursuing if you think the strata committee and strata manager are likely to move quickly on this to avoid orders not to do so.
232 Orders to settle disputes or rectify complaints
(1) Orders relating to complaints and disputes
The Tribunal may, on application by an interested person, original owner or building manager, make an order to settle a complaint or dispute about any of the following:
(e) an exercise of, or failure to exercise, a function conferred or imposed by or under this Act or the by-laws of a strata scheme,
(2) Failure to exercise a function
For the purposes of this section, an owners corporation, strata committee or building management committee is taken not to have exercised a function if: a) it decides not to exercise the function …
(3) Other proceedings and remedies
A person is not entitled:
(a) to commence other proceedings in connection with the settlement of a dispute or complaint the subject of a current application by the person for an order under this section, or
(b) to make an application for an order under this section if the person has
commenced, and not discontinued, proceedings in connection with the
settlement of a dispute or complaint the subject of the application.
So your owners corp is in breach of section 232.2 (a) because it has clearly and openly decided not to exercise its duty to issue a notice to comply. Your strata manager should have cautioned them that this might be the case.
This could turn out to be complicated so you might want to consider getting legal advice from a specialist strata lawyer …