Chilling hours are the total amount of hours a plant requires to be exposed to cold winter temperatures in order for them to break their dormancy and set flowers and fruit.
The chill hours are calculated by combining the number of hours the temperature falls below 7.2 degrees Celsius, but can be affected by the number of hot days which reduces the number of chilling hours received.
Accumulated chill hours provide signals to dormant plants that winter is over and they need to start coming to life again for the spring season. If a cold climate plant isn’t exposed to sufficient chill hours over winter, it will delay budburst, have poor flowering and fruit set too.
So how do you calculate your chill hours where you live?
You can work out your average temperature for your area in the coldest month (July). A great site to work this out is http://www.eldersweather.com.au/
Enter your suburb or town in the top right search button. Then click on the full climatology button around ¾ of the way down on the right. Look for the July long term averages and add the maximum and minimum temps together and divide by 2, this will give you the average.
Then use the below table to work out your approximate chill hours for your area.

The below map also gives a general idea of the climate zones around Australia but doesn’t take into account elevation or micro-climates.

Low chill areas- up to 300 hours. Only grow low chill varieties in these areas.
Medium chill areas- 450-650 hours. As a guide you can grow all low and medium chill varieties, but please ensure that you protect your plants from late spring frosts.
High chill areas- 650+ hours. You can generally grow all low, medium and high chill varieties but please ensure your plants are protected from late spring frosts.