Road Laws in QLD
If you are travelling at 80 km/h in a 100 km/h zone you must pull over (when it is safe to do so) and allow cars to pass
You should also be aware that 80kph is not a cutoff - no specific speed is mentioned in Section 83 of the Road Traffic Act 1995 - it is up to the discretion of the officer to determine if you are obstructing traffic. So, even if you are only doing 10kph under the speed limit, but you are holding up 10 or 20 vehicles, you may be Failing to show Due Care and Consideration and are liable to be pulled over and fined.
This is, as with most Australian Road Rules, just common sense. Be courteous on the road. Don't obstruct other traffic, and if they're overtaking you, slow down to let them pass safely!
If you speed up, that is actively blocking them and not only is in violation of Section 145 of the Australian Road Rules, it could be treated as road rage!
These laws may not be relevant in other states, but if you are travelling in Queensland you must let traffic pass!