Tasmania's $100-million-a-year abalone industry is unsustainable and heading for collapse due to overfishing, veteran divers say.

Key points: Abalone have not returned to some areas where they once thrived, say divers

Abalone have not returned to some areas where they once thrived, say divers A marine heatwave caused mass mortality of abalone this year

A marine heatwave caused mass mortality of abalone this year Quota levels are not allowing stock to replenish, claim divers

Over the last five years the industry has taken fewer and fewer abalone out of the water but the stocks have still not rebounded.

In fact, veteran divers said this year the fishery was worse that it had ever been.

The ABC has spoken to more than a dozen divers, each with 10 to 33 years' experience, who said the wild resource had been overfished, making recovery difficult or unlikely without urgent action.

All but one of these men spoke on condition of anonymity for fear of being denied work by commercial operators.

As one diver put it: "To put your face to a complaint is cutting your own throat".

But they want to sound the alarm about the state of the world's largest wild abalone fishery.

"The abalone stock on the bottom is just depleting, flat out," said one. "We've had downturns before but never this bad. It's the worst I've ever seen it."

"Over the last 12 months it really fell off a cliff, especially around the west coast," said another.

To illustrate, in some places where divers could land 250 kilograms in an hour a few years ago they can now only hope to get 40kg.

In some cases, abalone grounds have not just been thinned out, they have been wiped out.

"I'm going into gulches that used to have good fish in them that have got none. We've fished those gulches out," said one diver.

Marine heatwave takes its toll

These are lean times for abalone divers compared to the 1980s, when Tasmania's coastline was an underwater El Dorado, its shallows tiled with abalone that can be sold for more than $100 per kilogram in Asia, converting humble divers into millionaires and spawning an export industry worth $100 million.

These days, most would be lucky to clear $40,000 a year - about 30 per cent less than what they were earning 10 years ago.

And now a marine heatwave has dealt another blow, killing off stock and reducing some divers' expected earnings by half.

For more than 100 days over the summer, the water temperature rose by as much as 4 degrees Celsius above average.

CSIRO researcher Alistair Hobday said this El Nino spike had dwarfed more recent warm water events.

"Since satellite records began in the early '80s we've never seen an event like this. It's over double the previous marine heatwave," said Mr Hobday.

The problem for the abalone industry is that this kind of event could be the new norm, as warm water events occurred in 2009, 2010 and 2013.

"Tasmania's getting a window into its future this summer," said Mr Hobday.

Mr Hobday warned that any fishery should be prepared for more frequent warm water events and be more cautious about their harvest targets, leaving "more of an insurance population in the ocean rather than harvesting to the limit".

The Tasmania's Abalone Council's (TAC) chief executive Dean Lisson said the body was already taking this on board.

"We are very much aware of the affect of climate change on the productivity of the east coast abalone zone," he said.

"Concerns about productivity have been reflected in our prudent decisions to cut the east coast quota so significantly over the last five years."

'We're knife-edge fishing it all the time'

Year by year since 2010, the whole fishery's total allowable catch has been reduced to a current level of 1,700 tonnes, about half of what it was in 1985 when quotas were introduced.

But divers said that had not worked.

A unanimous point they all made was that the TAC and its sub-committee, the Fishery Resources Advisory Group (FRAG), had been too conservative and that more radical cuts were needed to ease the pressure on the fishery.

"We're knife-edge fishing it all the time and we've just pushed it too far," said one diver.

"We can't just hope it's going to come right."

The industry will be gone if drastic action is not taken, says diver Chris Pepper. ( ABC News: Gregor Salmon )

Chris Pepper, 62, who has fished for abalone all around the state for 31 years, was the only diver prepared to speak out openly.

He said divers had been raising the alarm about overfishing for 10 years but their warnings had fallen on deaf ears.

"[They're] not listening to the divers or they don't ask the divers," he said.

"We're the ones that are bringing it up."

Mr Lisson said quota levels were always a matter of debate.

"Some say we've gone too far, others say we haven't gone far enough," he said.

"It's an extremely large and complex fishery to manage."

But Mr Pepper said the decisions were not being made in the best interests of the stock.

"The Tasmanian abalone industry is in turmoil," he said.

"The stocks on the bottom aren't sustainable for the quota that's coming out. The TAC doesn't seem to be recognising that quick enough.

"The majority of divers that I can speak for are really, really worried through the mortalities and the warmer water, and the TAC are not.

"The TAC are not responding quick enough to the warmer water and the mortalities."

'Eyes on the bottom' carrying little weight

Mr Lisson urged the divers to air their concerns at TAC and FRAG meetings.

"It's a shame they don't come to industry meetings," he said.

"If he [Mr Pepper] and his mates are genuinely concerned about the fishery, they should make the effort to attend the FRAG meetings and have direct input into the annual stock assessment process."

The divers said they had been doing that for years yet only lip service was paid to their views on controversial issues like quota.

Sorry, this video has expired A comparison of the sea bed off the north west coast from 2011 to 2016. ( Gregor Salmon )

According them, their collective "eyes on the bottom" once formed the basis on which quota assessments were made.

Not any more. Now, they say, they are predominantly the hired help in this industry, equipped with little power and an ineffective voice.

"The last downturn we had it took 10 years to come right," said Mr Pepper.

"Even if it does start to come right it's going to take longer this time.

"If they don't do something drastic and start listening to people that are on the bottom swimming around it won't have an industry the industry will be gone."