Hi Guys,
I'm sorry to hear of the hassle you had, that does suck. I was thinking to myself about this and this may or may not be of assistance to you but read on in any case.....
My understanding of the road rules are that if a road is not speed limit posted, and is outside of suburban limits, ie has no street lighting, then the speed limit is 80kmh unless otherwise posted. You may be able to argue that one as a technicality and then argue that your speedo calibration is within tolerance for you to be over the slightly over limit and still read OK.
Just a thought. It may have been 100kph prior to the roadworks, then the speed limit of the roadworks was enforced to 60kph, but the "end of roadworks" sign should indicate the end of the roadworks speed zone. In the absence of a new speed limit, the rules should be reverting back to 80kph unless otherwise sign posted rule. This is my understanding of the rules from "back in the day", perhaps they've changed. :-(
Good luck whichever way it goes.
DJ