What kind of natural disaster could destroy Adelaide?

Very little could destroy the city itself but the outer suburbs are under threat from bushfires. It is not an earthquake zone, it doesn't get tropical cyclones, there are no nearby volcanoes (Mt Gambier would be the closest) and if the Torrens flooded, no-one would notice, it isn't very big. Probably the very worst they could get would be a tornado from a severe thunderstorm but to destroy the city you would need an F5 and Australia has not recorded a tornado more powerful than F3.

So far their has been no natural disater that has destroyed South Australia yet but the State Govenrment has done an excellent job in distroying it. Does that count?

A tidal wave or an earthquake

The closure of Centrelink. There is not enough Kleenex in the country for the tears of Old Coward.

A meteorite.