I believe this is Ligustrum lucidum (Broad Leaved Privet).
It's an invasive species and is unfortunately rather common Canberra because it was planted as a windbreak many years ago. Like many other well-intentioned decisions to introduce foreign flora and fauna into Australia throughout the years, this had unintended consequences (it grows aggressively in clumps of coppice like this and chokes out many native plants).
Is it possible to share some further images of the leaves (preferably close up) and, if present any fruits or flowers?
Hi there. Thanks for the id. There were no obvious fruit or flowers. We've cut it back for the time being ( we are the Friends of Mawson Ponds group on our regular Wednesday weed, water, monitor morning) so we'll keep an eye on it.
It is Celtis australis, nettle tree, not Ligustrum lucidum. The leaves are pointed and serrated. It was used as a street tree but is now known as invasive. Cut back and poison is the best treatment. It looks as though it could have been cut back previously and has regrown.
I saw some more Celtis australis on my walk today, but your group will no doubt find them. I did record Ligustrum lucidum (Large-leaved Privet) which is more important to get rid of.