Richie,
There are different categories of ILS. The basic, cat1 allows a descent to approx 200 feet above threshold, with visibility down to a minima of around 500 metres. The more precise is cat 3, where you can more or less put the thing down (or the auotopilot will!) down to a height of 0 feet and and RVR of just a few metres!! You confuse me a bit going on about octas. This is just a meaurement of how much of the sky is covered. You could have 8 octas (or overcast is referred to mostly) at 1000 feet, or the same at 100 feet, neither would be classed as fog, which means visibility less than 1000 metres. I'm sure melbourne has an ILS, but whether or not you could land depends on how low the vis is, the ILS equipment and ALSO what equipment the aircraft has. You could have cat 3 ILS equipment on the airfield, but the aircraft (and crew) may only be cleared for cat 1/2.
Hope this clears things a bit.
Simfly