As I understand it, in NSW, a Police Officer doesn't even have to look at their speedo, they can simply estimate the speed you are travelling and issue book you based on that. The only evidence I have of that is a 2003 report on speed enforcement:
Traffic infringement notices for travelling in excess of legal speed limits (or speeding fines) can be issued
following detection of excess speed in a number of ways: from stationary radars (including speed cameras);
through laser speed testing (LIDAR); from vehicle mounted radars in police cars – which can operate
whether the police car is moving or stationary; from speed checks using a police car speedometer; and from
an estimate of speed.
http://www.ombo.nsw.gov.au/publication/PDF/specialreport/Speedometers and speeding fines.pdf
I think there's a couple of factors to consider before you think about going to court, and information we need to know before making any kind of assessment.
1) It will be your word against his.
2) The speed limit in that location, if it's a 50 zone say, he's assuming you're doing AT LEAST 70kph which is a big difference. Unless you were absolutely positive you were doing the limit or below - I wouldn't bother.
3) Your previous driving record will be a deciding factor I'd think, if you've never been pinged for anything, it will most likely increase your chances.
4) You're speedo need only be accurate to 10% over 40kph, but if your car has rims that are too big, or massive tyres, it can throw that out somewhat - which will further damage your chances.
Based on only what you've said, *so far*, you've not got much hope simply because police are deemed as expert witnesses. You'd do well to get some actual legal advice and see what the precedent is here.