The first is doing 260-280. Though you can knock 20 off for speedo calibration.
As far as what you can do:
45+ is on the spot six month suspension if you are pulled over, fixed camera its just points and fine. At a guess past say 180km they could charge you for reckless driving, which could land you in jail.
Really though if you are contemplating these speeds the limit is not the law but the car, road and bravery. eg generally a road-car will not be happy or in any way safe past say 200km. Then the car will need to be more and more of a purpose built track-attack car past that. 300km is nations cup monaro kind of speeds (though 911 Turbos, etc will get up there).
Then there is the road as a general statement 150 is the fastest you will be able to go on a non-highway/airstrip. Airstrip or race track will be the place to find v-max.
Then there is driver bravery and skill. At these speeds in road cars and your skill level bravery is what you are relying on. And you will need it, steering control will be marginal as you approach v-max. Infact all control will be, the only thing you are doing is holding on to the wheel and holding the accelorator down. Slowing will take some skill as it must be gradual.