I fail to understand why the aircraft was anywhere near Mt Disappointment in what appears may have been marginal weather at best. Newspaper reports allege that Batman Park was the pickup point and Ulupna was the destination.
A direct track between those points runs parallel to the Hume Freeway leaving Wallan and Wandong about a mile to the West and Mt Disappointment about four miles East when abeam. Such a track avoids the high forested parts of the Divide but it does traverse Melbourne controlled Airspace out to about Kalkallo.
One way the track might have intersected the Mt Disappointment area is if the helicopters planned to avoid controlled airspace or didn't receive a clearance through it and tracked East to say, Viewbank before turning North and tracking direct to Ulupna for reasons of time and economy instead of following the VFR corridor, which they must have crossed. that would have taken them to the Kilmore gap and relatively safer terrain afterwards.
However this is just speculation.