To be fair if you are referring to the MIA VA/QF fog incident the BOM did have a TEMPO for low cloud on it all morning, including when the jets diverted. There was reported low cloud in the area, and the fog materialized pretty quickly from nowhere. Neither jets held for an hour before attempting a landing.
I agree with the issues at ports that seemingly should have met watch. Forecasts are for planning and at least the first 5 hours should be extremely accurate (as per the BOMs own statement). If there is fog nearby, put on a probability. It's better I have extra fuel and off load a bit of payload for nothing than plan min fuel on a good forecast only to be told at top of descent that there's suddenly fog. Forecasters seem to be confusing forecasting with issuing observations.
And yes, I've seen several occasions lately where the TAFs at SYD and MEL were amended as or after the weather deteriorated, not prior to.