I am not in Victoria but is the chair also resigning from the committee? I would expect that if the chair resigns, then the committee would elect a new chair from among its (remaining) members.
There might also be a casual vacancy on the committee. In the ACT, perhaps similar in Victoria, the committee can appoint someone it chooses to fill the casual vacancy. If the chair resigned from the committee as well, not just from the role of chair, then the committee could fill the vacancy first and then elect a chair, who could be the new committee member. That chair would be in the role until a new committee is elected at the next AGM.
If the chair resigns by putting it in writing, then that is correspondence that you might send to the managing agent for the OC records. Otherwise, I don’t see that the manager has anything to do with it other than being a hopefully well-informed source of advice about relevant processes in Victoria.
If the chair has just spat the dummy and wants nothing further to do with the committee, then the remaining committee would have to record something polite to that effect in its minutes and then get on with electing a new chair.
But it might be different in Victoria.