I'd suggest that the area 10-15 miles to the north which is a problem would be near the You-Yangs and a bit hilly which might be upsetting the reliability of the radar. Aircraft use that area to transit to the west just outside the Avalon airspace. If someone is up high near the lower limit of class C, they could potentially interfere with traffic for the south runway at Avalon. I've heard many times the Mel Radar operator calling aircraft in this area when a Jetstar aircraft is both inbound and outbound. If the lower limit was a fraction lower it might not be as much of a problem.