That's wishful thinking but not true. I've directly called a legitimate taxi company before and been scammed and had to pay over triple the price. This was after specifically discussing the flat cost that this particular trip had. Due to circumstances I'm not going to discuss here I had no choice but to pay it, but if I'd been in a different situation I would have left.
Regulation is very lax here. Plenty of stories. The most basic being they won't unlock the door when you flag them until hearing where you are going - legally they have to take ALL rides unless they are in a shift changeover. If it's really far, like to a farm, I think they can ask for the return fee too.
Also, a booking isn't. It's like that Seinfeld skit. A booking is just an agreement to put your job out at the agreed time. Doesn't mean anyone has to take it. I live too close to the airport for anyone to want to take it, apparently.
I don't know the situation in Sydney, but where I usually travel, it's the unlicensed taxis that overcharge and scam people, i.e., not the real taxis.
Licensed taxi companies charge standard fares.