Yes bolthead: The person who recorded the lowest temp ever recorded at YGLB knows.
It's -8.5 according to this website:
Goulburn historical averages, records and extremes But the website says it only covers from 1971 onwards. And it does not say what source it uses for the data.
I recorded -2 F in 1967 using a wall thermometer I purchased on the advice of Joe the Gadget Man, so I guess it's -18.889 C unless someone else has recorded a lower temperature.