This is a tall herbaceous perennial (ht 1-2m) from central and eastern USA, which flowers in late summer and early autumn in Sydney. It is typical of the prairie flowers from that country, which form tough basal clumps of leaves and send up tall spires of flowers. Coming from the Asterceae family of plants, Boltonia asteroides has clouds of tiny daisy flowers, coloured pale blue/lilac, white or pinkish purple. It prefers to grow in a sunny, fairly moist position with reasonable soil. The tall flowering stems are cut to the ground after blooming. It is quite vigorous once established, and the clumps need to be divided every three years or so in spring or autumn.
Postscript: my plant eventually succumbed to fungus during a wet spell in early autumn. I think it was planted in too moist a position. I have seen it do well in other Sydney gardens. Instead, I now grow a very tall pale blue Aster to give the same effect as the Boltonia.