Hibiscus trionum, Bladder Ketmia
It is always interesting when a new weed makes an appearance in the garden as you wonder not only as to how it got there but also its origins in the plant world.
Apparently this is a fairly widespread summer annual weed occurring along much of the mainland Australian coastline as well as inland. It is of Middle Eastern origin but is widespread in cultivated land throughout America and parts of Europe (eastern Spain).
I think this came from a bird dropped seed or from potting soil of a newly planted adjacent Lilly-Pilly. Either way it has a fairly innocuous presence as it is only about 17cm high and quite fragile in appearance. The pastel coloured flowers are quite pretty and give rise to the common name of 'flower-of-an-hour', while the seed pods are bladder or balloon shaped.