Vehicle access to the island is already under construction.

“The cracking we’ve seen on other parts of the river bank near Murray Bridge has begun on the existing mud track where they’re about to build the causeway,” said Mr Pederick, shadow minister for the River Murray.

“I visited this area over two years ago while drillers were taking core samples and it was apparent then the swamp was on two metres of sand sitting on at least 20 metres of soft mud.

“It doesn’t take much imagination to think of the disastrous possibilities. And locals told us that in a north-westerly blow, waves up to six-foot high surge across the lake shore to the narrow mud causeway and into the river on the other side.

“If that water pushing across is salt water, as Premier Rann has said, we’ll have yet another disaster on our hands.”