Highly likely there will be restrictions as the river is a public corridor and is deemed navigable i.e. it is used boats (in particular jet boats) and other craft. You have raised several key concerns such as health & safety and then there is the aspect of pollution and general impact on the immediate environment.
Control of rivers is complex with various government departments and regional Councils and local Iwi being either responsible or having an input to the management of different parts or aspects of it e.g the water itself, the riverbed. surrounding land, without necessarily there being one controlling body.
My personal view is that at the moment unless someone else has some spare time to volunteer to investigate it that the current team of event organisers will not have spare resource to pursue this but it could be something we could look at in the future.