The Age has an article about bike riders using parks when they are not supposed to. I thing the enforcement of these laws is fair enough. The parks in question have clear signs explaining what is acceptable and what is not.

The problem is that councils and Vic Roads don't do enough to encourage cycling on the road. Bicycles are banned entirely Brunton Avenue, which is a major approach to the city centre. Because of its low speed limit, narrow curbside lanes and lack of on street parking, Brunton Avenue is an ideal cycle commuting route. The alternative is for bike riders to use Yarra Park. But exposes pedestrians to conflict from vehicles and encourages bike riders to continue towards the city through the Fitzroy and Treasury Gardens.

If you don't want bike riders in the Fitzroy and Treasury Gardens, don't force them to use Yarra Park.