The Government has planned for a massive 1080 drop over South Island forests this autumn, in response to what's been called a widespread "mega mast".

The Department of Conservation (DoC) says there will be heavy seeding in New Zealand forests this year - the biggest in more than 40 years - with the most set for South Island forests.

That would provide a bonanza of food for native species, but would also fuel rodent and stoat plagues that threaten native birds and other wildlife as predator populations build up next spring and summer.

In response, the DoC's Tiakina Ngā Manu predator control programme, previously known as Battle for our Birds, will see more than 66,000 ha of trapping with the remainder - more than 900,000 ha - aerial 1080 operations.

"DoC is planning its largest-ever predator control programme for 2019/2020, at a cost of $38 million, to suppress rats, stoats and possums over about one million hectares or 12 percent of conservation land," Conservation Minister Eugenie Sage said.

She said it's a step up from the previous largest programme overseen by former National Party Conservation Minister Maggie Barry, of 840,000 ha in 2016 and 600,000 ha in 2014 ,when there were significant but smaller mast events.

The priority sites for predator control include the South Island's Kahurangi, Abel Tasman, Arthur's Pass, Westland, Mt Aspiring and Fiordland national parks, the Catlins and Whirinaki.