The public transport isn't great in St Ives. There are buses to Gordon and Pymble. However, pending where in St Ives you are, they usually take long routes and don't have great timetables.
There is a decent bus route during peak hour times that goes to the City via East Killara, East Lindfield and Roseville and vice versa which takes about 45 minutes.
Pymble and Turramurra have good train stations, albeit with not too much parking. Gordon has a greater amount of services as express trains stop there and some trains terminate there which can be annoying if you're wanting to go one or two extra stops but not the end of the world.