Noxious weed: Elephant dung has been linked to an outbreak of alligator weed near Hamilton.

Spreading circus elephant dung over his land turned into a jumbo-sized headache for one Waikato landowner after it was discovered the pachyderm poo was infested with alligator weed.

A spray contractor discovered the four hectare outbreak on the landowner's property, near Hamilton, earlier this year.

Inquiries by Waikato Regional Council biosecurity staff revealed the weed began sprouting after a load of elephant dung was spread on the land.

The potent poo was bought as a fertiliser from a visiting circus in Hamilton, in 2007.

The alligator weed was also found on a neighbouring property where the elephant dung was applied.

The council has declined to identify the sites so as to not deter people reporting infestations.

The outbreak prompted the regional council to check historic carnie sites across Hamilton.

No other infestations have been unearthed to date.

Waikato regional councillor and pest management committee chairwoman Laurie Burdett, said alligator weed was a dangerous pest plant that needed to be kept out of the Waikato.

"[It] can spread really quickly, taking over pasture, crowding out native plants and choking waterways, hurting both the economy and the environment," Ms Burdett said.

"This case involving elephants shows just how careful and vigilant people have to be to help keep alligator weed out."

Alligator weed has spread throughout Northland and is often found in cheap silage or hay.

It's possible the elephants consumed such feed.

About 60 per cent of what elephants eat is excreted virtually untouched.

Regional council biosecurity natural heritage group manager John Simmons said alligator weed had an extensive root system, which could reach down several metres, making it extremely difficult to eradicate.

The pest plant can also grow from any sort of node.

An alligator weed infestation near Cambridge took almost 10 years to fully eradicate.

Because the risk of accidental spread was high, Mr Simmons said people who discovered alligator weed were urged not to try to control it themselves.

Alligator weed is classified as a direct control species in the council's regional pest strategy, meaning the council will assume responsibility for eradicating infestations.