Police don't need you to agree that you were speeding. They will still give you the ticket regardless of your thoughts. If you disagree, you then take the matter to court. Clever Police officers will not say "Did you realise you were doing 100 in the 80 zone back there?". Because they won't want your answer to be "I thought it was 90 officer, I disagree." The Police don't want a confrontation or argument on the side of the road, thats what the courts are for.
A smart officer will generally say, "Any reason for you not abiding by the speed limit back there?". Most people will realise they actually were speeding by a certain amount and answer with many reasons or even say they don't know. Alot of people are nervous or pissed off and will then forget to even ask how much they were speeding. At this point, the officer will ask for the licence, go back to the car, write the ticket and then bring it back to you. Only then will you know your speed that they clocked you at. You will probably then begin to complain once you see the infringement details, but as I said, the courts are where you fight the ticket, not with the officer on the side of the road. He will advise that if you want to dispute the ticket, then the details are on the reverse side of the infringement notice.
This is based on WA. Unsure if Vic has different court system, infringement notice forms etc.