A plan to dump dredged sediment onto a sensitive wetlands area beside the Great Barrier Reef near Abbot Point would lead to much more slurry being pumped into the waters of the reef than officially estimated, a report has warned.

In its submission to the federal government, the environment group WWF cites expert advice that the project’s modelling underestimates by nearly 30% the amount of sediment and water that would be discharged into the ocean through a pipe from the wetlands ponds.

A total of 1.7m cubic metres of dredged seabed will be mixed with nearly 12.5m cubic metres of seawater to create the slurry, which will be stored within ponds in Queensland’s Caley Valley wetlands.

The seabed is being removed in order to expand the Abbot Point port, a coal export terminal near Bowen. The ponds will be constructed near new railway lines running from Abbot Point.

Brett Miller, principal engineer of hydraulics and modelling at the University of NSW, said it would be “very difficult” to achieve the amount of discharge into the ocean estimated by the Queensland government, which has put forward the plan.

The discharge would have to be nearly 30% greater to stop the ponds of sediment from overflowing into the surrounding environment, Miller states.

This could have potentially significant ramifications for nearby beds of seagrass, which are a vital food source for dugongs and turtles. Dumped sediment can smother seagrass and corals, effectively killing them off in some cases.

The Queensland government has submitted the dumping plan to the federal government as an alternative to a previous tactic of dumping five million tonnes of sediment into the Great Barrier Reef marine park itself.

Unesco, which monitors the health of world heritage sites such as the reef, raised concern over the initial plan due to its potential impact on the coral ecosystem.

Jon Day, a former director at the Great Barrier Reef Marine Park Authority, informed the WWF submission that the project assessed the impact on world heritage attributes based on out-of-date information and that mangroves, turtles and seagrass near Abbot Point should also be considered.

Further expert advice, from Prof Richard Bush of Southern Cross University, raised concerns that the impact of acid sulphate soils in the dredged material was not adequately assessed.

WWF said it was concerned that the ponds would be constructed in January, when they could be at risk from cyclones, in order to meet a dredging “window” that opens in March.

As reported by Guardian Australia, the Queensland government is preparing for work to start on the project from New Year’s Day. The federal government set aside just 10 business days for submissions on the new dumping plan. About 80,000 were sent via environment groups.

Queensland government documents on the project state that the Caley Valley wetlands are an important habitat of national significance for more than 15 migratory birds, collectively described as “superlative natural phenomena”.

The dumped sediment would directly wipe out 114.3 hectares of foraging habitat for birds, with a further 16.4 hectares lost due to increased activity and noise. More than 97 hectares of potential sea grass habitat would suffer “permanent and irreversible loss” from the dredging.

WWF’s reef campaigner, Louise Matthiesson, said the Queensland government had underestimated the impact of the “rushed” new plan.

“The sea grasses are only just recovering from cyclone damage and we can’t afford to lose any more,” she said. “The muddy plume from the discharge pipe would spread over a much larger area than is currently estimated. The impact of those fine sediments escaping into the ocean has been drastically underestimated.

“We absolutely welcome the decision to not dump at sea, but if they are going to be dredged then this is the wrong location. It’s a high value conservation area directly next to a world heritage area. This proposal is badly designed and needs a rethink. It’s taking big risks with the reef.”