You know where Poyntons Nursery is? There is a canoe club back towards Orrong Road near there with a floating platform. You can enter there. Alternatively just past Poynton's is another entry point that's a little messier.

Definitely suggest going upstream first. You can get quite a distance. Follow the main river as the side branch up further degenerates into almost nothing and you'll struggle to turn around.

You will travel past the back of the old munitions factory with a couple of spots you can pull in if careful.

If you go downstream you can get all the way to the Yarra but will need to watch out for other water craft. And then you have the fun of coming back upstream... :)