Obloid larvae

Description:

My Acacia mearnsii (Black wattle) died recently. It seems to have been hollowed out by some beetle larvae. They are obloid when small (15mm) but get cylindrical when larger (25mm).

Habitat:

Suburban back yard. A common local tree that appears throughout our forests.

Notes:

This species of acacia seems very vulnerable to many pests. They die suddenly and fall easily spreading their seed when they crash down. I often find a patch of thousands of new seedlings in the forest where an old tree has collapsed and I'm entertaining a theory that this is a strategy for reproduction.
I am guessing mostly likely beetles possibly longicorn borers (Cerambycidae)
No luck with likely species yet...