How do I save my Grevillea from a cockroach?

I was walking past a mature Grevillia in my front garden when I noticed a pile of very thin wood shavings around the base. I went looking and found a clump of the shavings a little way up the trunk and as I cleared it more and more came out until I revealed a sizeable hole in the trunk.

I turned the hose on and flushed out the hole and it took a lot more water than I thought it would, after the initial flush a pair of antenna poked its head out the retreated, I flooded it again and then after a minute out crawled what looked like a pretty sizeable wood boring cockroach.

Cockroach is out but the hole practically goes all the way from one side of them trunk to the other, another cm and it would be straight through.

Besides the hole the tree looks ok, it's flowering, there's new growth coming from the trunk but there's no way that hole isn't going to do something.

I was thinking of filling the hole, but I'm not sure what with or if I should treat it with something first to stop any rot or little cockroaches that might hatch.

Help!