What made me suspect it might be D. tasmanica rather than D. longifolia or D. revoluta were the quite distinct, regular prickles on the edges of the leaves. They are easily overlooked at the publication resolution (though visible on the uploaded higher resolution image) so I've uploaded a close-up image showing the prickles . While D. revoluta apparently has a subspecies with something similar (but apparently larger and less regular according to https://plantnet.rbgsyd.nsw.gov.au/), I haven't found a description (or image) of D. longifolia that includes teeth along the edges of the leaves. Have I missed something?
I agree the altitude might be borderline but still think there is a good case for D. tasmanica based on the leaf morphology.
Meredith Cosgrove (Photographic Guide to Native Plants of the ACT, 2014, p.184) says D. tasmanica occurs above 660 m. The altitude of my sighting is 674 m according to the camera GPS although more likely to be about 655 m based on the Lidar-derived topo map. CNM has three sightings classified as D. tasmanica in the National Arboretum at an altitude of about 620 m. They may have been planted, but it demonstrates that they survive at that altitude and that there is a local seed source (if the ID is correct for those). My sighting meets Meredith Cosgrove's (ibid) Key ID features for D. tasmanica: "Leaf margins and midrib underside edged in short, saw-like sharp teeth". I have uploaded an additional image of my sighting that confirms both of these.
I wouldn't take notice of planted specimens at the Arboretum. Your find is a wild specimen. Burbidge & Grey says it occurs in forest gullies and alpine woodland. But maybe your are right. It would be an exceptional find. Maybe we can revisit when it is more mature.
I don't know whether the Arboretum ones are planted or not, but you're right that it will be interesting to see how it develops and we'll be keeping a close eye on it.