Scientists fear unique corals that fringe Australia’s remote Norfolk Island could disappear because of a triple threat of disease, El Niño and a federal government plan to dredge a neighbouring shipping channel.

A failure to manage sediment and pollution washing into bays from cattle farming, cleared land along with wastewater has been blamed for widespread disease and outbreaks of algae over the corals.

One expert said most of the island’s corals were likely to be species that have not been formally described by science, and would be unlikely to recover if lost.

“We could rapidly lose the coral reefs and we won’t get them back,” said Prof Bill Leggat, a coral expert at the University of Newcastle who has been monitoring the island’s corals and the disease outbreak for five years.

Speaking from the island 1,600km north-east of Sydney, Leggat said since March there had been a three-fold increase in diseased corals during of one of the longest-running coral disease events recorded on Australian reefs.

Water quality was a key problem, he said, where pollution and sediment would wash into the coral lagoons during heavy rain, promoting disease and algae.

“The main issue is the nutrients that increase coral disease and then algal growth that puts more stress on the corals. It’s frustrating – we should be able to fix that,” he said.

He said “white syndrome” disease on coral started with a white spot that gets larger and larger, killing the coral flesh and leaving the visible white skeleton behind.

Norfolk Island, with a population of 2,200, has reefs running for about 2km around the three adjoining bays of Emily, Slaughter and Cemetery.

The island attracts tens of thousands of tourists each year and was one of Australia’s earliest penal colonies before welcoming in 1856 the Pitcairn island descendants of mutineers of the famed HMS Bounty.

Prof Tracy Ainsworth from the University of New South Wales, a member of the team monitoring the island’s reefs, said between 30% and 50% of corals had shown symptoms of the disease over the past five years.

Water monitoring carried out by CSIRO on the island has suggested pollution was likely to be coming from cattle manure, wastewater management – including septic tanks – and fertiliser.

Ainsworth said the developing El Niño in the tropical Pacific would risk elevating summer water temperatures, causing corals to bleach and potentially die.

The federal government’s infrastructure department gained environment approvals in April 2025 to dredge a channel to improve access to Kingston Pier, where many island supplies and tourists arrive. Work could start later this year.

The problems with water quality, the plan to dredge near the reefs and the El Niño had created a perfect storm for corals, Ainsworth said.

“That is everything that you can do to kill corals. It is too much for corals to survive,” she said.

Neil Tavener – known locally as Snowy – is a 73-year-old lifelong island resident who swims over the corals most days. Before retiring, he worked for the island’s administration on water quality and public health.

“The lagoons and the corals are the jewels in Norfolk Island’s crown – they’re priceless,” he said.

“It has taken maybe a couple of hundred years for white man to stuff it up, but it is slowly happening.”

Dr Tom Bridge, a coral taxonomist at the Queensland Museum and James Cook University, is researching the corals of Norfolk Island and Lord Howe Island, about 900km west.

He said about 40% of the corals on Norfolk and Lord Howe were likely found nowhere else. They were isolated from other corals, which meant they had little chance to re-spawn because of a lack of neighbouring reefs.

“These corals are at a really high risk of extinction – but it is a silent extinction because they’re not even described [in the scientific literature],” he said.

“The populations are quite small and isolated from each other. If you lose them, they are not coming back.”

A spokesperson for the federal infrastructure department said the Kingston Pier project was a “critical upgrade” to deepen and widen the channel allowing “all-tide harbour access” to accommodate “larger commercial, cruise, and fishing vessels”.

The department intended to award a contract for works in August and rock wall repairs were expected in coming months, with dredging to follow.

The project was approved under national environment laws with strict conditions, a statement said, including limiting dredging to a 0.5 hectare footprint, disposal of spoil on land and real-time water quality monitoring.

Independent specialists would oversee monitoring “to protect the surrounding reef and marine environment”, the statement said.

A spokesperson for Norfolk Island regional council said septic systems, wastewater, stormwater and other activities all needed “ongoing management to protect water quality”.

New strategies to manage waterways and cattle grazing were both in development, the council said, and a working group with representatives from multiple government agencies had been formed to co-ordinate water quality management.