Let me say this. At the planning stage and up until departure, if there was no fog forecast on the adl taf then an alternate would not have been planned.
Secondly mildura was probably an enroute adequate port for the sector flown, which can have fog forecast on it and still be a legal plan.
Mad I know! But thems the rules. So here lies the problem when an aircraft actually needs to use an enroute adequate port, and the prob fog becomes a reality , Houston we have a problem.