When did Australia change from degrees Fahrenheit to degrees celsius?

Answer 1

Australia, along with many countries uses both systems. There is no legislation anywhere that says the people must use one system or the other.
The scientific and technical community everywhere uses the SI ( Le Système International d'Unités) system of units. Though that system uses the Kelvin as a base unit it has a different starting point but is exactly equivalent to degrees Celsius or Centigrade. Which are useful units in the everyday world.
It is important that we agree on definitions of certain things, especially in the physical world. Hence we have the SI system.
In the ordinary workaday world it matters little if you say it is 80 degrees outside, no one is going to think you mean 80 degrees Celsius or Kelvin. One is too hot the other is too cold.
It would be ideal if everyone used the same units.
Calendars are another subject of debate. Most people accept the Gregorian calendar. Many religions use archaic and (more) inaccurate ones Lunar, solar, luni/solar and I just made it up ones. Though they will use the Gregorian one in everyday life. (Ask them when they are due a tax return)

Answer 2

Because it has been adopted as the standard unit of measurement in most countries, therefore, to ensure ease of trade, Celsius was adopted.