Limited info to make an ID on but I'll go along with KCR on some sort of oak.
Take a look around. Acorns generally don't travel far unless you've brought in some mulch.
There are many different banksias and each species has a different shaped leaf. Some have smooth edges on the leaves - B. integrifolia, B. marginata. Others have leaves with pointed serrations - B. serrata. B. ericifolia has small, narrow leaves but there are many other shapes and sized leaves in the WA banksias. Which means that there is not really any 'typical' banksia leaf shape.
The young branches and the way it buds out IMHO do indeed point to a Banksia. The leaves of Banksia solandri come closest, but that is a native of WA, Stirling ranges, rather far from where we are! Since you are a member of the Canberra Bonsai Society, you can get help from some of the most knowledgeable members concerning native bonsai. Just take a sample of the shrub to the next meeting.
Or maybe Gavin or Grant will see this post........
Lisa
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Took a few more pics. Maybe that's a bud or flower forming on the ends? The underside isn't grey, almost the same green as the top of the leaves. Neil, there isn't any oaks close by but we do have a lot of bird activity in the area - so long as I'm vigilant with the hose on the neighbours cat (note: only when it's misbehaving).