Australian AIP (as at 24 Apr 97) states:
Published ILS CAT 1 DA and visibility minima are available to all aircraft except that:
a. visibility 1.5KM is required when HIAL is not available; and
b. visibility 1.2KM is required unless
(1) the aircraft is manually flown for the entire approach using a flight
director or the aircraft is flown to the CAT 1 DA with an autopilot coupled
(LLZ and GP); and
(2) the aircraft is equipped with a serviceable failure warning system
for the primary attitude and heading reference systems; and
(3) high intensity runway edge lighting is available.
Where do these visibility requirements come from? The next illustration will answer this.
Assuming a runway point of intercept of 800ft, a DH of 200ft some trigonometry reveals:
Tan 3o = 200/x
x = ~3817ft (1163m)
Where x is the distance from the RPI to the point on the ground directly below the aircraft. 800ft equates to around 243m so the distance from the runway edge to the aircraft is approximately 920m. Looking in AIP AGA the distance between HIAL bars is listed as shown in the diagram. Working forward from the aircraft the usual visibility of 800m will be in the band shown depending on the exact position of the bars you may or may not see bar #1. You should "CLEARLY" see bars #1,2, and 3. Depending upon aircraft configuration and seating position you may or may not see bar #4.
1.2 KM visibility will allow you to see the RPI and this gives added visual reference to account for failure of aircraft systems, inaccuracies during the approach, and reduced definition of the runway if the edge lighting is unavailable.
1.5KM visibility will allow you to see the VASIS and is required because of the black hole that would occur if the HIAL was unavailable. The tendency to adopt a shallow approach is negated if you follow the VASIS, even if the HIAL is working it is a good idea to use the GS information until the VASIS is in view.