They will need to be taller than that - all the marinas
outside the river were okay and indeed even a marina at the river mouth was fine. (rivergate)
Many of the floating pontoons may have come from well upstream where the river heights were possibly 10 meters higher than usual. For example there were many pontoons on the river at Jindalee, usual river height here is around two metres - give or take, on Friday well after the peak (which was thursday) it was well above eight metres, I cant access the historical data back beyond Friday at the moment but we can assume that the poles would need to be fairly tall based on that data alone.
Even then I am not sure if the poles would take the drag etc and not give way, there are a number of poles in the river where pontoons used to be that are bent over. Its going to be interesting to see what the Commission of Inquiry says about that along with many many many other issues.
Of course this is a once in 100 year event and of course we all said that in 1974 as well.