The weather for one day at one location can't be used as an indicator of anything. There are a great many variables that affect the weather, when these come together in the right combination they cause weather anomalies. In this case a very hot day, it could have been a very cold day, a very wet day, a very windy day etc.
The reason for stating it was the hottest day in 150 years is because that's as far back as the instrumental temperature record for Sydney stretches (159 years if we wanted to be pedantic).
In relation to global warming we need to look at a much wider picture taken over a much greater period of time. For this reason climate scientists use a base-period of 30 years against which temperatures are compared. Far more indicative of global warming is that across the whole world the last 6 months was the warmest such period on record and that 2009 is set to become one of the hottest years on record (top 5 almost certainly, quite probably 3rd).
