I also agree with Stevecro, otherwise there is no point in having a notice and agenda. As a general matter of the laws of meetings, you can’t have a resolution/decision unless it is on the agenda (note this can vary depending on the circumstances, for example you may have to make a decision on something in an emergency, or at late notice).
Here the owners have an interest in the purported resolution. Of more concern is that the meeting was said to be closed and the EC then “made” the decision after that – it seems to be very deliberate. I would be writing a letter to the EC asking them why they did that.
I also agree with Whale – the relevant provision is section 65A of the Act, the owner has altered/added to common property and require approval or should be compelled to remove it.