The Act will specify that some sorts of decisions require a special resolution. If the Act does not say what class of resolution, assume an ordinary resolution. It is not up to the EC or OC to decide. The Act tells you what class of resolution is required.
75% is, I think, not quite pedantically what is required. It is correct in effect most of the time. I think it is actually specified as 1) a majority in favour and 2) fewer than a quarter opposed of those taking part in the meeting, in person or by proxy. In principle you could have 60% of those in the meeting in favour (>50% but <75%), 20% opposed (<25%) and 20% abstaining. In that case I think the motion would pass. If it were 60% in favour and 40% opposed and no-one abstaining it would fail.
That is the logic of how it works in the ACT except that it is fewer than a third opposed rather than a quarter. If NSW is not quite like that someone can correct it!