We get asked this question alot. The right warmth will vary depending on a number of factors like where you live, if the home insulated, do you use heating and mostly each individual feels the cold differently.
Most quilts sold in department stores are a 4 blanket warmth, its the most common warmth as it covers the whole year for most Australian cities for most people.
Many rural areas are more likely to get the colder nights, places like Orange NSW or Ballarat Vic, places like this that get below zero for much of the Winter usually require the 6 or 7 blanket warmth.
The tropics obviously don't require the same warmth as the southern states, we would usually recommend the 1 or 2 blanket warmth. The 2 blanket warmth quilt is also the ideal warmth for the southern states Summer.
Below is a list of which warmth quilt is likely to be needed for the various temperatures around Australia. These are an approximate as I mentioned everyone feels the cold differently. If you feel the cold go for a quilt 1 blanket warmer, if you don't feel the cold so much you might need to go 1 blanket warmth less.
1 Blanket: ideal for night temps between 20 to 30 degrees
2 Blanket: ideal for night temps between 15 to 25 degrees
3 Blanket: ideal for night temps between 10 to 25 degrees
4 Blanket: ideal for night temps between 5 to 20 degrees
5 Blanket: ideal for night temps between 0 to 15 degrees
6 Blanket: ideal for night temps between -5 to 10 degrees
7 Blanket: ideal for night temps between -10 to 5 degrees
Now crazy people like ourselves have a 7 blanket warmth on our bed all year round. I said to my wife during the Summer that we really should have a Summer quilt (2 blanket). I was told no get a 4 blanket for Summer??? She likes it to look full and fluffy (who doesn't) so practicality doesn't play apart in our house.
So its not to say that you cant use a 7 blanket for all year or that for some the we know a 2 blanket is still too hot for Winter, everyone is different but for most the temperature range above is about right.
Hopefully this guide helps in deciding the right warmth for you.