Unsure what this was. Around 12mm in length and whilst the antennae were constantly active the insect didn't move, not even for closely passing ants. Thanks to Marcus Leach for helping with this sighting.
Possibly a species of Rhipicera (but head/thorax area doesn't look right), current Australian reference seems to be ZHENYU, Jin; Escalona, Hermes; Slipinski, Adam; Pang, Hong (2013). "Phylogeny and Classification of Rhipicerinae (Coleoptera: Rhipiceridae) with A Review of the Australian Taxa". Annales Zoologici. 63 (2) 275-317. Available on line with some good images