The legislation is absolutely clear on this and it is spelled out here in Schedule 2 of the Act.
In brief summary, any committee meeting has to be notified to all owners three days before the meeting and that notification must contain an agenda of the meeting. The committee may only consider items that are on that agenda. The meeting must have at least half the committee members there (before “proxies” are considered) for decisions to be valid.
To prevent this happening again, you might write to the committee and tell them to stop all work immediately unless they want to be charged individually for the work done.
They should immediately convene a proper meeting under the terms set out in Schedule 2 of the Act to confirm they have the authority to proceed with the work.
In the meantime they should issue a full and unequivocal apology to all owners for not having followed the correct procedures and undertake not to ever do this again.
Now, there are two things that will probably happen when you do this.
1. They may, indeed, convene a meeting to retrospectively authorise the work.
2. You will be removed from their Christmas card list.
You might also warn them that if they don’t convene a proper meeting and do things properly, you might apply to NCAT to have them replaced by a compulsory appointment of a strata manager.