from VicRoads website:
Exemptions
There are some circumstances in which a person may be exempt from wearing a seatbelt, such as when the driver is reversing the vehicle, or is engaged in the door-to-door delivery or collection of goods or garbage and is travelling under 25 km/h.
Obviously if you are at a give way sign or red light etc you wont take off ur seatbelt, that comment was ridiculous.
The topic was about him getting out of his driveway and driving down the street. As long as its under 25km/h from what he was doing then it should be fine. If you were to contest this within court you would ask what a reasonable man would do in that situation. For all you know he maybe in a rush that morning for work. Getting out of his driveway and putting on his seatbelt whilst driving down the street wouldn't be a problem in the court of law at all. I believe that their already is precedent for this type of case, all you have to do is search on google for case files pertaining the similar situation and contest this it in court and claim damages (for the money lost from work for attending court) maybe even negligence from the police officer for giving you a fine. Most cops will save they have you on tape, they tend to twist their words to make it sound different to intimidate you. For all you know he must of had a vid capture of you reversing out.