Adam,
When the weather deteriorates below a certain level, aircraft require an alternate airport - another place they can fly to if they are unsuccessful on their approach.
If the fog was not forecast, or it is a payload limited flight, it may well arrive near Melbourne without this level of fuel.
So, while the aircraft MAY have got in, given the fog hasn't gone below the landing minima, the forecast of fog means they need another aerodrome to fly to, and it is quite possible they did not have that.
It takes a lot of additional fuel to carry Sydney as an alternate for Melbourne.
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