The flowering time for this (hybrid) variety is from late spring through to autumn, so it should be flowering for a while longer yet, although a lot of plants are going into an early autumn mode due to the heat we have had lately, which in fact should suit this sun loving Australian shrub very well.
I think what you suspect (overwatering) could be right, as the shrub is fairly drought tollerant, allow it to dry out a little between waterings, keep it in a sunny spot and slow release ferfiliser as recomended. it may also need some winter protection, depending on your location. Good Luck Tbird+