As a starting point, any “decisions” made at those meetings are not invalid. However, certain decisions can be challenged. There are a few steps you need to consider. Note you haven’t said what the size of your scheme is, as the requirements are slightly different.
You have mentioned the Chair, Treasurer and Strata Manager but not the Secretary who is responsible for notices of meetings, have you told the Secretary that they are breaching the Act?
1. A notice of a Strata Committee meeting must be given to each Committee member and owner at least 3 days before the meeting. The notice must include a detailed agenda, which would include any proposed decisions.
If the scheme is more than 100 lots, then the notice can be placed on the notice board.
2. If owners have an objection to a proposed decision on the agenda, one third of them by unit entitlement can give written notice of that objection to the secretary before the meeting, and any decision will have no force or effect.
3. Going to the actual decisions that might be made at a Committee meeting, the type of decision can vary. So the Committee might make a decision about something relatively minor or routine, such as doing a regular pest inspection. Or they may resolve to enter into a contract to do major works that cost a lot of money.
The latter type of decision is one that could be challenged, if the owners were concerned that proper processes were not followed. In the case of a Committee decision to proceed with major works, the owners could seek an order to stop those works proceeding, or call an EGM to reverse the decision, refer below.
4. Another factor is the scope of matters that can be decided by the Committee. If you are concerned that the Committee is going to continue in this way, then the owners can restrict the types of decisions they make, including the type of decision and an amount. So you would include a motion on the agenda at the AGM to that effect.
5. Another option if the Committee made a decision that was of concern to the owners is to call an EGM to reverse the decision.
Note that if the Committee had entered into a contract pursuant to a decision, and the owners wanted to reverse that decision, this could create legal difficulties. The other party to the contract is entitled to rely on the validity of the decision and the authority of the Committee to enter into the contract, in the absence of notice to the contrary.