I looked into it when i used to run the junior matches and Simon is correct that a freshwater license is needed above the railway bridge.
It could be worth it if you are after mullet though as they tend to congregate in the pools here.
To access, you can park in the carpark near the rspca building down Marsh Road and walk through the kissing gate to the cob.
You will see the mullet all the way along the back of the cycle track to where the cut drains into the river, when they are in.