for a dramatized look at the canker see the attached. I think Electra Klein use to live in Sydney.
It does not look like seridium. I think it looks like malpruning is a factor--if you shear the lower branches and leave the upper branches wide, they can shade out the lower ones.
I would prescribe an overall thinning/reduction/"dropcotching" and cleaning out all the dead stuff to get some light and air moving in there. Then look for the sticky streaks on the stem.
Betcha a nickel they aren't there to any signficant extent.