It took only 15 minutes to reel in two three-to-four kilo European carp at a dam in Bega on Friday.
One of the fishers, senior biosecurity officer South East Local Land Services Andrew Michelin, was shocked at their size, describing his catch as "monsters".
"We've seen photos of them one metre long," he said.
The LLS began the Far South Coast European carp monitoring program in November last year in a response to the high number of the invasive species reported in the region.
"It's surprising and alarming," Mr Michelin said.
"We we're stunned to realise the numbers were a lot bigger than we thought."
So far the fish has been found in the Bega and Brogo Rivers and has been reported to be in the Brogo Dam.
Mr Michelin said the carp were bottom feeders that separated sediment at the bottom of waterways causing a deterioration in water quality and a reduction in light penetration.
This reduction in water quality smothered plant life, reduced other species's abilities to find prey - including birds - and they competed with native fish.
The Australian Museum states European carp have reached 60kg overseas, while in Australia they can weigh up to 10kg.
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A native of Asia, extensive introductions have helped make it the world's most widely-distributed freshwater fish.
According to the museum three strains were introduced to Australia: an ornamental strain near Sydney (1850-60), a Singaporean strain in the Murrumbidgee (1876) and a hybrid "Boolara" strain in Victoria (1961).
The latter two strains interbred and this species is now a major pest in many inland streams in NSW.
"One of the reasons they were introduced was to give the Australian environment more of a European look," Mr Michelin said.
He said the aim of the LLS's monitoring program was to assess the impact of carp in the region and get an estimate of their numbers, then look at suitable control options and follow with a three to five year management plan.
While a herpes virus is currently being developed as a control option, Mr Michelin said it was still at least 18 months away before the virus's future would be determined.
Other options included carp fishing competitions as well as electro fishing.
This involves emitting an electrical pulse into the water which stuns the carp and causes them to rise to the surface to be collected, but does not kill other native species.
If you see or catch a European carp, Mr Michelin said you can report it on the FeralScan app for mobile phones and tablets or contact the LLS biosecurity team in Bega on 6491 7800.
"If you catch them, you should be euthanising them as humanely as possible, but do not release them back into the wild," he said.