The Great Barrier Reef Marine Park Authority initially thought a dredging project may cause long-term, irreversible harm, new documents show.

There will be a push in the Senate on Tuesday to revoke the approval of the Abbot Point coal port expansion.

The Greens will ask for a vote after it was revealed the Great Barrier Reef Marine Park Authority (GBRMPA) initially wanted to reject a plan to dump three million cubic metres of dredge spoil in the marine park over several years.

In the draft report to the federal environment department – obtained by Greenpeace under the Freedom of Information – GBRMPA said the project had the potential to cause long-term, irreversible harm.

But in January, it approved the plan, which will turn Abbot Point in north Queensland into one of the world’s largest coal terminals.

Greenpeace campaigner Louise Matthiesson says the organisation wasn’t aware of any significant changes to the project which would justify a major change in GBRMPA’s stance.

When approving the application, GBRMPA imposed 47 conditions, which chairman Russell Reichelt says are the strictest conditions that have ever been put on a port development.

He said the documents obtained under FOI were preliminary working drafts and were never submitted.

“As such they do not represent the views of the agency,” he said.

“It’s important to note that the draft permit assessment was conducted before stringent conditions … were put in place by the environment minister.”

Mr Reichelt has previously said GBRMPA’s preference was for the spoil to be dumped on land but the authority failed to persuade the North Queensland Bulk Ports Corporation to do so.

However, he said land dumping was difficult given the port was surrounded by valuable wetland and bird habitats and the authority was satisfied with the offshore dumping plan.

Environment minister Greg Hunt said he was advised the proposal put forward for offshore disposal was the best option available.

“I was advised that the previous Labor government had already ruled out all other options,” he said.

Bulk Ports, which will undertake the dredging, has said it could possibly make the water cloudy during a short period and may damage seagrass but it’s unlikely to affect other flora and fauna.

They say dumping the spoil in the water will be less damaging to the environment than depositing it on land.

But green groups argue they’re opting to dump it at sea because it costs less.