A TAF is updated, usually every 6 hours.
The initial TAF (according to AJ) had no thunderstorms forecast, but as you say, it was updated getting closer to the mainland, the forecasters added thunderstorms to the forecast.
As they get closer they can also make use of a TTF... Trend Type Forecast, which is updated every 30mins and valid for 3 hours. They would have checked the MEL TTF prior to Sydney, and if it had Thunderstorms on it, then they would divert into Sydney.
The weather that is present in a city is largely irrelivant... it is what the forecast says that is.
Even if the MET people think there is a 30% chance of thunderstorms in this situation, the aircraft diverts. Thunderstorms are a funny one, because it will usually remain on the forecast for a long time, as the forecasters will see big buildups that could, at any moment, develop into a TS. They often never form, or never affect the airport, but if they COULD, then they go on the forecast.
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