Despite the signs, I'm still pretty sure it's nothing more than a courtesy. I don't think I'll be getting in any trouble with the law for preventing someone from exceeding the speed limit...
Vic law is must keep left when signed or at any speed limit above 80 km's. It's very much the law, although since speed cameras can't book you for it, it doesn't seem to be enforced very often.
I also wont move left for an idiot doing 20-30 over the speed limit, drive the monash daily in the 80 zones, i'll sit 2-3 seconds behond the car in front usually doing slightly more than posted limit, dickhead tailgate me, I wont move out of the right in this situation - I don't have to either is the road is busy. One the open roads, i always stay left unless overtaking.
Victorian Road Safety Road Rules 2009 rule 130.
http://www.legislation.vic.gov.au/Domino/Web_Notes/LDMS/LTObject_Store/LTObjSt6.nsf/DDE300B846EED9C7CA257616000A3571/2A57410F82D46474CA257A2C00192B8A/$FILE/09-94sra006%20authorised.pdf
130 Keeping to the left on a multi-lane road
(1) This rule applies to a driver driving on a multilane
road if—
(a) the speed-limit applying to the driver for the length of road where the driver is driving is over 80 kilometres per hour; or
(b) a keep left unless overtaking sign applies to the length of road where the driver is driving.
Notes
1 Length of road and multi-lane road are defined in the dictionary.
2 Part 3 deals with speed-limits.
(2) The driver must not drive in the right lane
unless—
(a) the driver is turning right, or making a U-turn from the centre of the road, and is giving a right change of direction signal; or
(b) the driver is overtaking; or
(c) a left lane must turn left sign or left traffic lane arrows apply to any other lane and the driver is not turning left; or
(d) the driver is required to drive in the right lane under rule 159; or
(e) the driver is avoiding an obstruction; or
(f) the traffic in each other lane is congested; or
(g) the traffic in every lane is congested; or
(h) the right lane is a special purpose lane in which the driver, under another provision of these Rules, is permitted to drive; or
(i) there are only 2 marked lanes and the left lane is a slow vehicle turn out lane.
Penalty: 3 penalty units.