Frank: At AY during the day most tracks drop out at about 8000ft. Sometimes, if the atmospheric conditions are right, you may get a sniff down to 6000 or 7000ft. However, it is in no way usable for separation as it is intermittent and 'jumps' around a bit. During the night, any traffic going overhead or slightly south of AY FL140 and below gets treated as a non-radar aircraft for separation purposes as they pop in and out of coverage along those routes, sometimes for periods in excess of 5 minutes.
Benalla, IIRC, is pretty useless below 6000ft. With the right conditions in the atmosphere, alignment of the planets, etc, you may see them below that slightly. Again, not well enough to use for separation. I don't think there's any way in which the radar can be tweaked to look lower, but I'm no boffin so I may be wrong.
Cheers,
NFR.