While I am a world away from Mount Hotham, the technology remains the same. At this point my instincts tell me that using a LiDar equipped drone might yield some results if there is an area of high probability. Scanning with LiDar allows the trees and undergrowth to be removed from the scan. The resulting image is very accurate to the point even burial sites that have settled are visible. It is such a vast area to search, but one can scale it down by determining how far the couple could have walked in a day (point A to point B), put a pin in the place last seen, measure out that distance from point A and draw a circle. 80% probability they are within that circle. So say you feel they could have walked 30 km in a day, you would draw a 60 km circle around the place last seen or known to have been.