Volunteer divers have killed almost 47,000 crown-of-thorns starfish on the southern Great Barrier Reef in just seven days, breaking a record in the process.

Thousands of the starfish were discovered only months ago eating their way through parts of Swains Reef, which lies 250 kilometres off the central Queensland coast.

Gladstone charter operator Bruce Stobo led the group of 25 volunteer divers on the nine-day mission.

"They tell me, unofficially, that it's the most amount of starfish that [have] been killed in a single trip in that time," said Mr Stobo, who donated his catamaran to the mission.

Similar missions have culled up to 30,000 crown-of-thorns starfish.

Queensland Parks and Wildlife detected the infestation during a survey in November last year, but this did not prepare the divers for what they saw.

The volunteer divers returned, tired but jubilant, after a nine-day culling mission on the Swains Reef. ( ABC Capricornia: Inga Stünzner )

"Everyone was absolutely surprised," Mr Stobo said.

"We had professional divers on board who've been working up north, and one in particular who's been doing it for eight months full-time, and he said it was the most that he's seen in one area."

Shocking problem

Crown-of-thorns starfish were so thick in some areas that they were layering on top of each other to get to the coral.

Bruce Stobo donated his catamaran for the nine-day crown-of-thorns culling mission. ( ABC Capricornia: Inga Stünzner )

The divers operated in groups of 12 and each did three dives a day, with each dive lasting up to an hour.

Starfish were injected in the shoulder with bile salts, a job easier said than done as the divers had to contend with swells and currents.

"When you've got that number of crown-of-thorns in the water, they die and they let go and they end up like tumbleweeds in the water, so you've got to be careful that you don't put your hand down on ones that are there and the ones that are in the swell," Mr Stobo said.

"For the first couple of days, we did training on board so we were getting our technique right and after day two, we got into the thick of it and the weather was fantastic, so it allowed us access to some huge aggregations."

Mr Stobo said the culling trip was never going to kill all the crown-of-thorns at Swains Reef, but it did show them how big the problem was.

Queensland Parks and Wildlife were also part of this expedition, surveying the reef and identifying where the concentrations of starfish were.

"By no means have we got to the front of it, but we are aware that we have large numbers out there, and with a combined effort, we can certainly put a dent in it," Mr Stobo said.

Funding welcomed

Mr Stobo welcomed the Federal Government's $60 million funding announcement to tackle crown-of-thorns starfish in the Great Barrier Reef and hoped some of that money would be funnelled to the southern end.

One of the volunteer divers on the mission, Penny McCracken from the Sydney Institute of Marine Science, suggested some funding could be put towards finding ways in which to herd the starfish to one area in order to cull them.

"You are limited in how much time you can spend underwater and the reef is immense, so that herding would be helpful," she said.

This technique has previously worked for culling cane toad tadpoles.

Injecting starfish 'quite hard work'

At the moment, injecting the crown-of-thorns starfish with a single shot of bile salts is the most effective method, but it is not easy.

"It's quite hard work when you get up to the 500th crown-of-thorns starfish in a 60-minute dive, so technology with the guns would also help speed things up," Ms McCracken said.

She said she had been diving on the Southern Great Barrier Reef for 20 years and was shocked at the number of starfish in one area where they had piled on top of each other.

"When I first started, we would may be find one or two crown of thorns during a one-hour dive."

Penny McCracken and Wayne Hubbard, photographer, look at film footage of the mission. ( ABC Capricornia: Inga Stünzner )

"We spent two days just trying to get a handle on this part of the reef," she said.

"It is overwhelming, but with more boats and more divers we might be able to reduce the damage."

Although some areas of the reef are devastated by the crown-of-thorns starfish, other areas remain pristine.

"It wasn't all covered in crown of thorns, and that reminded us why we were there," Ms McCracken said.