If your old Christmas tree is pine then don't dump it in the countryside, the Department of Conservation says.

Photo: MPI

By dumping an old pine tree, you risk spreading an invasive species already covering a swathe of New Zealand's back country, high country and landscapes.

Wilding pines - an invasive species - cover about 1.8 million hectares of the country, which is about six percent of New Zealand's land. Overall, species cover an area larger than commercial forests.

Department of Conservation technical advisor Keith Briden said the trees can take over sites, virtually replacing native plants and animals and render farmland unproductive.

"We have important landscapes ... and the tourists come to New Zealand to see our unique landscapes, plants and animals. Wilding conifers completely take those over.

"It was estimated [in 2015] that within 20 years, 20 percent of New Zealand would be affected by wilding conifers."

Photo: Department of Conservation

Last year, then conservation minister Maggie Barry announced new funding to combat the spread.

In Budget 2016, an extra $16 million a year was announced over four years.

Initially, conservationists aimed to treat one million hectares, but the department eventually treated 1.2 million hectares of conifers in 2017.

Mr Briden said people should take their old trees to landfill, or cut them up for green waste, but do not dump them in the countryside, especially if they have cones.

"Don't take them out in the countryside ... those cones can have some seeds."

Several species of pine are classed as invasive weeds in New Zealand and they are spreading at a rate of 90,000 trees per year. The trees are hardy and difficult to remove once established.

The next stage of the eradication programme covers 371,00 hectares in the Mackenzie Basin, Canterbury, Lammermoor in Otago and Mid Dome in Southland.