A banned aquatic weed, Salvinia, has been found in Northland.
The region was previously free of the fast-growing fern, which forms dense mats which block and degrade waterways, affecting flood control and blocking irrigation systems.
Nationally, Salvinia is found in waterways around the North Island and as far south as Christchurch.
The Northland Regional Council has confirmed two cases in the region both in garden ponds in the Hikurangi area.
It was first confirmed in the region in 1980, but was thought to have been eradicated. Council biosecurity manager Don McKenzie says the floating part could quickly re-establish in Northland's warm climate.
"This thing will double its size in a couple of weeks, especially in warmer waters. Fortunately it's out of our major river systems but that's what we fear.
"If it got into river systems, especially slow-flowing ones, it would cause mayhem."
Don McKenzie says anyone who spots Salvinia should report it to the Northland Regional Council or the Ministry of Agriculture's biosecurity department.
Moth spread sparks concerns
Another pest, the Gum Leaf Skeletoniser Moth, has also sparked concerns after spreading to Whangarei and Kerikeri.
The pest caterpillar's larvae feed on the leaves of eucalypt trees causing significant damage to plantations.
The moth is found across the North Island and a parasitic wasp was released in Auckland last year to combat it.
Don McKenzie says Northland is now trying the same approach.