Marama Davidson and Julie Anne Genter will face off in a debate at the Green Party's summer policy conference today.
The two women are running to be the party's next co-leader, after Metiria Turei's resignation at last year's election.
Politics lecturer Bryce Edwards of Victoria University says it will be difficult for the pair.
"In the Green culture, you need to be quite civil - you can't really get too aggressive," he told Newshub.
"That's quite hard if you want some way to show you're better than the other candidate. You can't do it via negativity."
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Being the party's first time in Government makes this leadership selection process different.
"One of their co-leaders, James Shaw, is a minister in the Government, and to some extent that means his hands are tied a bit. He can't properly lead the party because he has these other portfolios he has to manage."
He says picking the right person will be vital to keeping the Greens above the 5 percent threshold required to stay in Government.
"They need to get someone that will be able to keep the Green brand out there. So often when minor parties go into these coalition arrangements, they suddenly disappear."
Mr Shaw announced on Saturday the party's MPs would no longer accept kickbacks and corporate hospitality.
"This is another small way that we can show that the Green Party is committed to doing Government differently, and doing Government better," he tweeted.