"She must have just been a quiet, latent supporter of ours."
It is also the largest donation to any political party since 2014, when former Conservative Party leader Colin Craig gave $355,000 to his party.
Helm said most of the Green Party's fundraising was based on small, regular payments.
"We do have a quite comprehensive fundraising programme but a large bequest like this is extremely unusual for us.
"We tend to get a lot of small and medium-sized donations from people who perhaps have some disposable income but aren't the very wealthy in society."
Other large donations to the Greens include a one-off payment of $60,000 by Les Mills International chief executive Philip Mills in 2014.
The Greens raised $408,000 last year, of which $237,000 came from its own MPs under the party's tithing system.
That total was more than Labour, who raised just $279,134.
All donations of more than $15,000 have to be publicly disclosed in party's annual returns, while those of more than $30,000 must be disclosed within 10 working days of receipt.
Donations over $30,000, 2015-2016
• Green Party - $283,835, estate of Elizabeth Beresford Riddoch (August 2016)
• Act Party - $51,500, Dame Jenny Gibbs (March 2016)
• Conservative Party - $130,000, Colin Craig (March 2016)
• Conservative Party - $39,300, Colin Craig (December 2015)
• Act Party - $33,589, Dame Jenny Gibbs (December 2015)
• National Party - $60,000, Gallagher Group (December 2015)
• Act Party - $33,000, Dame Jenny Gibbs (May 2015)
• Act Party - $32,000, Alan Gibbs (May 2015)
• Green Party - $30,080, James Jenkins (April 2015)
