I grabbed the bug spray to spray some bugs (obviously), and ironically discovered this nest. The wasp that did this stings and paralyses spiders, builds them into a mud cell and then lays eggs on them. The hatching wasp larvae then have a living feast to consume. The poor spiders get slowly eaten alive, unable to move. The larvae then pupate and hatch into wasps to continue the cycle. Gotta love Aussie animals.