The Green Party is more than a quarter of a million dollars richer after an out-of-the-blue donation.

It's come from the estate of Nelson woman Elizabeth Beresford Riddoch, whom the party didn't even know, and who wasn't even a paid-up member of the Greens.

The payment appeared on the Electoral Commission's website, where parties have to declare all donations over $30,000.

The full sum of Ms Riddoch's donation is $283,835.99 and the transaction was made by her lawyer at PL Law.

Greens general manager Sarah Helm says it came as an enormous surprise when it arrived in the bank account.

"The donation came out of the blue, we weren't expecting it," she says.

"Obviously we're really happy, I mean it's very sad that she's passed, but we're very happy we're going to be able to use this to strengthen our ability to protect and enhance the environment," says Ms Helm.

The party has contacted Ms Riddoch's family to thank them.

"We're very grateful to them. She was a quiet supporter of the Greens, not a paid-up member. It was in her will and they were acting on her instruction," she says.

Newshub.