Investigations into a cluster of cane toads found north of Sydney last year have revealed a bigger population than originally thought.
Nineteen cane toads were found sheltering under a sheet of tin on a private property at Mandalong on NSW's Central Coast in October.
Another two of the invasive pest species were later discovered at Lake Macquarie.
The Department of Primary Industries (DPI) said a total 51 cane toads had now been uncovered.
The DPI's Quentin Hart said DNA testing suggested they all arrived at the same time.
"I know it seems unlikely that 50 toads can be introduced at the same time but the toads are only just getting larger than 50 grams," he said.
"They could have all been introduced at the same time via a truck or horse trailer or something like that."
During the initial discovery, DPI investigators said it was possible the cane toads had been breeding on the property.
Mr Hart now said that seemed unlikely.
"We have had DNA analysis completed last week, which is suggesting that the most likely scenario is a single introduction event some time last year," he said.
The department's biosecurity team has also conducted extensive water searches and put up listening posts to detect any adult cane toad breeding calls.
"So far over summer, we're yet to detect any breeding activities," Mr Hart said.
The DPI considers cane toads to be a serious non-native pest, which can devastate local wildlife and environment. If swallowed, the toxic toads can be poisonous to animals.
Surveillance to continue
The species is most prevalent in Queensland and northern NSW following a deliberate release in the 1930s to control pest beetles on sugar cane.
The DPI has established a cane toad containment zone around NSW's north coast to mitigate further spread into the state.
The Mandalong outbreak is understood to be the first significant outbreak beyond the buffer zone in NSW since 2010 when the amphibians infiltrated Sydney's south.
Around 650 cane toads were found during that outbreak.
Mr Hart said it could take some time before the Central Coast outbreak area was officially declared cane toad-free.
"Even after we've gone a year or two without finding something, we will maintain some level of surveillance and some level of community awareness," he said.
"We take a real precautionary approach."
The DPI biosecurity team will conduct consistent surveillance of the Central Coast area until June.
Efforts will ramp up again in spring as it gets warmer.
