This is an Australian Native Hibiscus (Alyogyne huegelii), some times called Lilac Hibiscus. It has been claimed that it thrives in the most desolute of places (Simpson, A.G.W. (1982) Bush Garden Handbook, Rigby Publishers, Australia). That goes a long way to explaining why it grows so well in my garden.
Actually, I managed to kill off the original one in my ‘garden’. This one self seeded and thrives on the bank of the lagoon, literally just on the other side of the property boundary. I guess it left home.