It's hard to imagine where these signs are.
I cross the commons twice a day, once in each direction, using the only, as far as I know, legal bicycle path (ie the broadly North-South route from Narbonne Avenue to Cedars Road). I've looked, fairly carefully, and I have seen no signs, aside from the normal blue bicycle route signs. Unusually, I don't think there are even any "No Cycling" bits where the footpaths and cycle paths cross, which are quite common in places like Hyde Park which also allows cycling on some routes.
Given that there are previous cases where the lack of no cycling signs has seen prosecutions of cyclists overturned [1] , I can't see that a speed limit with no, or extremely poorly signed signage is going to have any effect.
Footnote [1]: The case involved No Cycling markings on the footpath not being adequate, as I recall. I'm sure someone more familiar with such things will be along shortly.