They are called empirical evidence - datasets of temperature observations confirming there has been no statistically significant warming for over 15 + years.
The first major limitation in estimating an Australian - mean temperature prior to 1910 is that there was no national network of temperature observations.
Additionally, the Bureau maintains multiple temperature datasets — analysed in different ways — to provide a consistency check on the accuracy of temperature observations.
[v] Gillett N.P., V. K. Arora, G. M. Flato, J. F. Scinocca, K. von Salzen, 2012: Improved constraints on 21st - century warming derived using 160 years of temperature observations.