Organisers of the Sparke Helmore NBN Triathlon Festival canned the swims between Queens Wharf and Horseshoe beach, not because of health concerns but rather the debris in the water.

The Newcastle Herald reported on Thursday that polluted floodwater was thought to have caused the largest recorded fish kill in the Hunter River.

All marine life downstream of Morpeth died in what was thought to be a ‘‘blackwater event’’, when rotting vegetation robs the water of oxygen.

Water samples taken by the Department of Primary Industries last Friday showed that oxygen levels in the Hunter River were increasing.

Triathlon event director Paul Humphreys said they were unable to get a definitive response from NSW Health 

about water quality related to the fish kill.

However, he said the pure quantity of debris in the water by the weekend forced them to cancel the swims for the children’s and women’s races on Saturday and the main event on Sunday.

‘‘No one was prepared to give us a straight answer,’’ he said.

‘‘It just wasn’t safe.’’

Organisers replaced the swim legs with a second short run to maintain three-parts to each race.

More than 4000 spectators lined Newcastle Foreshore for the 20th annual event, which this year attracted a record number of entrants because of an extra race.

Nathan Miller took out the main event followed by locals Tim Lang, of Eleebana, and Elliott Collins, of Warners Bay.