They were on radar.
The VFR aircraft was slightly right of cleared route, but at about 15NM South deviated further to the right. The radar paint disappeared a short time later. Terrain shielding around Cairns is a regular occurrence, especially in this area.
Also, VFR aircraft in Cairns area are often maneuvering due terrain and weather, you can't constantly be checking all is OK.
As for IFR cleared at visual level at pilot request, what are we supposed to do?
Our system does not give terrain alerts for VFR aircraft or for IFR aircraft cleared visually.