The Coalition MP George Christensen has admitted he was wrong to support the dumping of 5m tonnes of sediment into the Great Barrier Reef marine park and has said he will push for alternatives to the plan.

In an open letter to readers of the Whitsunday Times and Whitsunday Coast Guardian, Christensen said: “Politicians don’t often say they got it wrong, but here it is: I got it wrong.”

The federal member for Dawson, an electorate which lies adjacent to the Great Barrier Reef, said he “didn’t foresee the angst the dumping of dredge spoil in the Great Barrier Reef marine park would cause tourism operators and the residents of the Whitsundays”.

Christensen has supported the expansion of the Abbot Point port, near the Queensland town of Bowen. The expansion, for increased coal exports, involves the controversial dredging of 5m tonnes of seabed, with the sediment to be dumped within the marine park.

It has emerged that scientists at the Great Barrier Reef Marine Park Authority warned against the dumping, only to be overruled. Unesco has also expressed concern over the dumping, which was approved in January.

Opponents of the plan claim it will smother coral and seagrasses, which are a vital food source for turtles and dugongs.

The federal environment minister, Greg Hunt, has insisted the dredging and dumping will have little impact on the reef. However, Hunt has also promised a “line in the sand” that will prohibit any further dumping within the marine park.

Christensen’s letter states the MP has “started talks with the owners of Abbot Point about land-based options for the disposal of the dredged material. They’ve agreed to re-examine all land-based options before proceeding with any works.”

He added that if a “viable option emerged, I will ensure that the spoil is dumped on land, not at sea”.

A Department of Environment assessment report released in December suggested that alternatives to offshore dumping “would involve significant expenditure”, with North Queensland Bulk Ports (NQBP), the Abbot Point project overseer, estimating that onshore disposal would cost between $120m and $460m.

Felicity Wishart, the campaign director at the Australian Marine Conservation Society, said she was happy with Christensen’s change of heart.

“I’m surprised but pleased,” she said. “George has previously dismissed the environmental concerns but he’s done a turnaround which is very welcome.

“What we’d really like to see is a commitment to ban dredging and dumping because the dredging itself will be damaging. That said, we recognise some dredging will go ahead and so we need to identify land-based sites where we can put the sediment.”

The North Queensland Conservation Council has launched a legal challenge against the sea-based dumping. NQBP has yet to identify an exact site for the dumping, despite the plan being approved by the federal government.