A huge banner unfurled by protesters at a Mackenzie dairy farm reads 'Stop Dairy Expansion'.

Forty five activists arrived at the site before dawn in freezing temperatures, with many chaining themselves to diggers and trucks.

The machinery is being used to excavate tussock land which is home to the world's rarest wading bird - the native black stilt.

Greenpeace sustainable agriculture campaigner Gen Toop says Greenpeace isn’t against farming, it's against bad farming.

She says the new mega farm is a shameful example of how rules supposed to protect rivers and the environment from industrial dairying are failing.

Toop says the protesters, including an 88-year-old, have been asked to leave by police.

But she says they've decided to stay and risk arrest to stand up for what they believe in.