I have no idea what this is, but I have seen them on a number of occasions. Always a single one sitting on what looks like a spider egg sac on a grass stalk, although they don't otherwise look very spider like. They are tiny - only about 1 mm. The last picture is a different individual from the others.
I can see a single head capsule on each cocoon, not a stack such as U. lugens produces. Also U. lugens feeds on Myrtaceae, not Poaceae. But of the Nolidae, only Nolinae have black hairy collars, so I think this is a Nolinae species. Of the 75 named Australian Nolinae species, I only know the food plant of 6 species, but sadly none of these feed on Poaceae.
I have added photos of the moth that hatched out of this cocoon. I have seen many of these cocoons and they have only ever had one head capsule. They are usually, but not always, on grass stalks, but there are plenty of eucalypts around that they could have been feeding on.