If the item isn’t on the agenda, it shouldn’t be discussed.
Better expressed that “if the item isn’t on the agenda then no decisions should be made about it”.
It is fine to discuss it, just don’t go making decisions.
“Importantly, the committee is bound by this agenda and not able to vote on a matter unless it is specifically listed. This requirement applies regardless of whether a matter is discussed in ‘general business’ or at any other point in the meeting. It would generally be the responsibility of the chairperson to declare a motion invalid if proper notice has not been provided.”
Rod Stowe (Fair Trading Commissioner Ref: FTMIN14/1685)
The same would apply to a general meeting.
Although it is highly unlikely that the lack of an agenda item denies capacity to make a decision (Sher Global Enterprises Pty Ltd v Owners – Strata Plan 31758 [2018] NSWSC 1057)