I do not believe this applies to Victoria, you are allowed to drive more slowly but not abnormally slowly in the circumstances, so I am assume that 70 kph in an 80 kph zone is not abnormally slow:
125 Unreasonably obstructing drivers or pedestrians
(2) For this rule, a driver does not unreasonably obstruct the path of another driver or pedestrian only because—
(1) A driver must not unreasonably obstruct the path of another driver or a pedestrian.
Penalty: 2 penalty units.
(2) For this rule, a driver does not unreasonably obstruct the path of another driver or a pedestrian only because—
(a) the driver is stopped in traffic; or
(b) the driver is driving more slowly than other vehicles (unless the driver is driving abnormally slow in the circumstances)