Now WUWT, following JoNova, is pouring scorn on BoM, saying basically that they are making it up, since they don't have a record. But there is a very good reason why they don't have a record. It wasn't then a BoM site. It was Air Force, and they get the records from them. And the RAAF has its own priorities.
However, the need for the change, and the amount, is obvious if you just look at neighboring stations, and the BoM program did. I'll show this below the jump.
BoM has all the unadjusted data you need, starting on this page. Ask for the kind of data (monthly, mean min etc), with Amberley in the matching towns. Under Nearest Bureau stations, unset the "only show open" button, and it gives the nearest stations with that data. I want stations with data from 1975 to 1985; enough to see what is happening around 1980. The nearest with this data are Ipswich, Samford and Brisbane. I didn't include Mt Glorious because it is on a mountain top; the others are pretty much on the level.
I then subtracted the monthly means for that decade, to remove seasonality. So here is the graph:
It shows the fitted lines, with trends over the decade shown in the legend. Note that the red Amberley curve is pretty much above everything before 1980, and below soon after. The other stations track each other well. Here is tabulated the trends in °C/century. Notice that one of these is not like the others.
Update. To show the contrast more clearly, and following a suggestion from Victor Venema, here is a plot of the same data, but with the Ipswich values subtracted - ie relative to Ipswich (chosen because it is by far the closest to Amberley):
The fact that Amberley crosses the axis during 1980 is now much clearer.So I added an adjustment of 1.4°C after Jan 1980. Let's see what it looks like now:
Interesting. Amberley is now back tracking the others, except for some disturbance during 1980. And the slope matches the others. But it's irregular during 1980. That means I haven't guessed the date quite right. I've adjusted upward too early. About August 1980 looks right. Here we go:
Yes, that did it. Now tracking the three neighbors very well. And yes, the slope has gone from -15.6 to 3.51, right in the mid-range of the neighbors.
Where did I get 1.4°C? It's a linear calc - to bring the slope back to mid-range. How does this affect the whole trend for Amberley? It increases the trend by 2.86 °C/Cen. Apparently the BoM adjustment was 3.5°C/cen. I'm sure they looked at more stations.
So there you go. The change happened in August 1980, and it dropped temperatures artificially by