We've been measuring temperature since Galileo's time. At least for Europe. It is just a matter of how much global coverage you have and the uncertainty range resulting from fewer, and less modern, and hence less reliably accurate records. Scientists consider there is enough global data to do a global estimate from 1880.
For Australia, the coverage is fairly sparse and without screens prior to 1910, so if you want a fairly accurate measure for Australia overall then 1910 is the start date. That doesn't mean there isn't data available prior to then, but there is much higher uncertainty associated with that data and it can't be considered an Australian continent wide record.
Take it from 1920 if you like. It doesn't change the reality. Warming since the early 1900's if not before. The deniers masticated and crowed for years on the 1998 'pause' being the end of warming, and they were wrong. They'll be wrong again.
