It is the same in the ACT. The remaining committee may (not must) appoint an eligible person to fill a casual vacancy until the next AGM when a whole new committee is elected.
In the ACT, at least, don’t know for NSW, the chair of the committee has a vote as a member of the committee and also a casting vote if a committee vote is split 50:50. Depending on what you think of the chair, you may or may not want to let the chair know!
It could be good politics to call for expressions of interest from among the owners or it could be quite reasonable to just invite someone who got substantial support but just missed out on being elected at the AGM. It might also be reasonable for the 4 remaining members to just carry on till the next AGM bearing in mind that the chair could break any deadlocked vote (in the ACT at least).