theres a heap of these tiny coccoon/cases made from plant matter and silk. On bottlebrush, i didnt see the actual grub. Assuming they're the same animal.
I think these might be Tortricids. Spilonota constrictana carry the 'nuts' from Melaleuca/Callistomen about like hermit crabs, joining 2 together when the caterpillar grows bigger. This may be the early stage before they are big enough to carry a 'nut'. https://moths.csiro.au/spilonota-constrictana-walker-1881-2/