yes, majority of bulb types are available, including the HB4 in the
100 series and H4/H1 combo in the twin headlight housing 80 series.
Patrol GQ and GU use the standard bi-xenon hi/lo single H4 bulb. The
not-so-common halogen bulb types, such as that of the H2 bulb (as used
in Hella 1000's and Cibie Oscar 'super slim' spotlights for example)
and the Osram JCD bi-pin (as used in all Lightforce spotlights) are
not available in a HID bulb. Or at least I have yet to find a
manufacturer who makes them... Or is willing to make them.

As far as I recall the 80 and 100 cruisers use switch-to-ground in
their headlight circuitry, (same as old EA81 subaru wagons and
Brumby's), which is the reverse to that of most other common vehicles
on our roads. Although saying that Hyundai Excel's use some weird
reverse polarity between lo and hi beam. (if anyone is bored one day,
go look at one). If you grab a normal kit for your 100 series, you'll
need to either drive another relay from your existing lighting
circuitry to drive the ballasts, or use a diode and cut'n'reverse the
polarity of the wiring. A lot of kits that I have seen (even el-cheapo
chinese rubbish) have details or at least mention this. Apparently 100
series cruisers also have some sort of loading sense on the hibeam
circuit, thus a normal kit will ignite the hi-beam dash indicator when
in fact lo-beam is on. A resistor of some value is also required.
There are kits available specifically for the 100 series which I'd be
inclined to grab for ease of installation.
Some dude on ebay is selling cheap 55w HID kits specifically for the
100 series. Another dude in Melbourne is selling 100 watt HID kits for
the cruiser. Expensive, but the 100 watt metal halide HID's would be a
gazzilion times brighter than that of the standard factory halogens.

More info:
ADR 77 and ADR 78 is around 130 pages of utter crap of what is and
what is not legal of HID on Australian roads.
You need washers on automatic tilting / adjusting headlight housings
which need to be connected to a sensor between the rear of the chassis
and diff. HID bulbs can not be longer than a certain amount, can only
produce a certain amount of lumens within a certain range, must be a
max of 35 watts, can not be any more than 4300k in colour, must be in
a proper E marked housing. Must be Euro light pattern, US and Japan
spread patterns are illegal.  It goes on and on... It gave somebody
somewhere a job for a quite while. Doesn't mention a thing about using
HID bulbs within spot lights or 'auxially lighting systems'.

Early cheap HID kits didnt have much in the way of UV protection on
the bulb. Which metal halides produce a lot of. UV can deteriorate the
headlight housing chrome reflector within a year or three. Bulbs in
newer kits that meet whatever ISO standards must not trasmit more than
umpteen % of UV. No idea what this is though.

I could go on and on but wont. I haven't written this much in the past
week on aus.cars.offroad since about 2003...

bugger. Yes, H7 is also available.