The law about keeping as far as practicable to the left is quite clear (TORUM-Road Rules : applicable in 5 states of Australia)
What is "practicable" may well be debated by some.
"
Division 2 Keeping to the left
129 Keeping to the far left side of a road
(1) A driver on a road, other than a multi-lane road, must drive as near as practicable to the far left side of the road.
Maximum penalty—20 penalty units.
(2) This section does not apply to the rider of a motorbike.
(3) In this section—
road does not include a road-related area.
Note—
Section 129 requires the rider of a bicycle on a road, other than a multi-lane road, to drive as near as practicable to the far left side of the road. "