A Senate inquiry into how the Great Barrier Reef is managed has heard that coal dust has spread across the World Heritage area.

The authority that manages the Great Barrier Reef assessed alternatives before issuing a permit to allow millions of tonnes of dredge spoil to be dumped in the marine park, a Senate inquiry has heard.

Federal senators are touring Queensland this week to examine how Australian and Queensland governments manage the reef, as UNESCO threatens to list it as a World Heritage site in danger.

Representatives from the Great Barrier Reef Marine Park Authority (GBRMPA) were on Wednesday questioned over a decision to permit a port authority to dump dredge spoil in the marine park as part of a project to expand Abbot Point coal port.

Biodiversity, Conservation and Sustainable Use general manager Bruce Elliot told the committee alternatives, including dumping the spoil onshore, were assessed.

“I think our staff did a very good job at identifying all of the potential issues, potential impacts and the potential effects that could occur,” he said.

“That is why we came to the conclusion if Abbot Point was to be permitted … it would need to be done under strict conditions in order to make sure the potential impacts were not realised and it was managed in a sustainable and acceptable manner.”

However, Mr Elliot said GBRMPA still took the view that alternatives could possibly provide a better environmental outcome than offshore dumping.

“But I wouldn’t be able to comment on the environmental negatives of what those would be and of course they need to be taken into account by the decision makers throughout the process.”

The authority’s chief executive, Russell Reichelt, told the committee dredging impacts could be significant but were localised.

He said the Commonwealth had been handed a draft long-term strategic assessment by GRMPA which addressed a range of issues including dredge spoil management.

Mr Reichelt said a reef program implemented a decade ago was starting to show positive results and the past impacts of the tourism, agricultural and farming industries had been significantly reduced.

“I am confident that there’s been good work happen with the strategic assessment and it does put us on a different footing in terms of managing environmental impacts in the future,” he said.

Earlier on Wednesday, James Cook University Professor Terry Hughes told the committee a “damning” new report out of the university shows coastal sediments offshore of the Hay Point coal port are contaminated with coal residues which exceed toxicity guidelines.

He said the report also found coal dust had spread hundreds of kilometres from ports.

The Senate enquiry finished hearing evidence on Wednesday.