Grass-carrying Wasp nest

Description:

The trunk of a dead dried black wattle tree with borer holes. Most holes on the tree were set deep in tunnels with a woody overhang and appeared to be ovoid -but I could be wrong. What was interesting was the holes were tightly stuffed with what looked like grass. The grass was dry so they could have been there for a while.

Habitat:

Spotted on a dead black wattle (Acacia mearnsii) in a nature reserve - Baluk Willam.

Notes:

I thought this was the work of a passerby, but holes high up on the tree also had grass in them. I tried to tug at the grass and some came out after much tugging - the grass appeared to be bent and twisted at the point where it was pulled in.
Mystery solved ! My husband helped ID this one - it is a nest of a sphecid wasp. The female wasp lays eggs in tubular holes on wood made by beetle larvae. She places items of food ( like insects) in the holes for her emerging larvae and plugs the holes with grass.
Family: Sphecidae