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The SFO is looking into donations made to the Labour Party in 2017.
Labour has been blindsided by the Serious Fraud Office announcement saying it does not know exactly what is being probed.
However, it did note the party told the media in February the two men who were then being investigated over donations to National had also given money to Labour.
At the time Labour Party president Claire Szabo said both were included as donations in the party return filed in the respective years.
Zheng Shijia donated $1940 in 2018 and Zheng Hengjia donated $10,000 by buying a piece of art at a silent auction in 2017.
The two men, along with Jami-Lee Ross and Zhang Yikun, face charges laid by the SFO.
In a statement, the SFO said it was conducting four investigations in relation to electoral funding, and a fifth was now before the courts.
The other cases involved respectively the New Zealand First Foundation, Auckland Council, and Christchurch City Council.
The latest developments mean in the run-up to the election only ACT and the Greens are not linked to any investigation.
ACT leader David Seymour said it raised questions about the country's reputation of being the least corrupt in the world.
"Perhaps most worrying is that you've allegedly got the same people donating to multiple parties, that doesn't sound like partisan activity, that sounds like external interference in our democracy," he said.
Seymour wants loopholes closed, but said ultimately it is up to politicians to make the right decisions.
"The electoral laws are there for a reason, to safeguard our democracy that so many people around the world wish they can wake up living in and if we take it lightly, if we drop our standards, then in a very real way it is at risk," he said.
The Green Party would not comment with an investigation underway other than to say that the Green Party has a long standing policy of reforming donations in New Zealand to increase accountability and transparency.
Currently every person who gives more than $15,000 must be reported by the party.
Anonymous party donations below that amount but over $1500 must also be declared.
Overseas donations are capped at $50.
Otago University law professor Andrew Geddis said political funding rules have been left fairly unchanged for about a decade.
"Parties have got used to operating with those rules in place and the question is whether they have found ways or think they might have found ways that those rules can be run right up against the edge of," he said.
Geddis said it showed that perhaps change is needed.
"What this does indicate is a time to step back, look at how those rules have been written, whether they are still fit for purpose and just really reassure ourselves that the system is working like it ought to," he said.