An email from Perth pointed out the plentiful mild weather lately. I see that Perth Airport 9021 is used in ACORN so I have had a look-see at all the raw monthly max data from CDO – (December data up to 16th so far). 
The first thing that hits me is a step-change coincident with the June 1994 install of a temperature probe (AWS). For larger version chart. Yet the BoM ACORN version for 9021 has no adjustment at 1994. The other point I would make is that UHI would have been steadily affecting this site as Perth housing & suburbia expanded producing warmer breezes. I can look at ACORN data later.
Here is a chart of raw monthly min data from CDO. Less noisy than the max. 
For larger version chart.
Here is a chart of annual mean T plus annual Range (mean T is the metric for “IPCC Global Warming/Climate Change”) 
For larger version chart.
“Yet the BoM ACORN version for 9021 has no adjustment at 1994.”
Come on Warwick it is not like they were competent enough to know their own faulty equipment caused the warming and chose to leave the error there.
Oh wait sorry. Yes it is. See quote that shows they knew.
“There is some indication of a small
(less than 0.2°C) increase in diurnal temperature range, most likely because of the faster response time of automatic probes relative to mercury-in-glass thermometers. This finding is consistent with international experience (Trewin, 2010).”
cawcr.gov.au/technical-reports/CTR_049.pdf
But it seems to be a bit bigger than what they fessed up to. Do you know what the minimums show since they owned up to a diurnal range error? Perhaps the politically correct way of saying known design fault is “not fit for purpose”.
I note that in the two years prior to 1994 AWS installation, the average RAW max at Perth Airport 9021 was 23.85C and average min was 12.70C.
In the two years after 1994 AWS installation, the average RAW max was 24.77C and average min was 13.22C.
During the two preceding years there were 235 days of rainfall and in the following two years there were 240 days of rainfall.
So in RAW, max up 0.92C and min up 0.52C, despite five extra days of rainfall.
In ACORN 2.1, the two years prior were max 24.15C and min 11.88C. The two years after were max 25.07C and min 12.43C.
So in ACORN 2.1, max up 0.92C and min up 0.55C, despite five extra days of rainfall.
So ACORN adjusted the temps and added a bit of extra warming to minima.
Australia’s temperature history is one of instrument change and adjustment change, with little climate change and UHI a probable factor.
Perth just had its hottest Christmas on record. To make things even, Darwin just had its coldest.