The article actually says Labour would be a big loser, because a significant portion of their donations (which are already much lower than National/Act) come from various unions.

That’s one of the recommendations from the electoral review quoted in the article, it just didn’t make it to the headline:

The panel said there was a “low level of public trust” in the private funding of political parties. It had two key recommendations to address this:

• Ban businesses and organisations like unions from donating.
• Limit the amount a person can give to any one party to $30,000 in each electoral cycle.

There’s a third problem though, which is that third parties don’t have to be transparent about their funding. The Taxpayers Union registered with the electoral commission to state their intentions to campaign in the election, but because they are not campaigning for themselves they don’t have to disclose their donation sources. It’s basically the idea of a Super PAC that the US has.

Act has to publicly declare all their donors over a certain value. So instead, you make a new organisation, have them pay for all the ads and other campaigning, and because that organisation is not a political party themselves they aren’t subject to the rules. Act can simply point their potential donors at the taxpayers’ union who do the dirty work.

For the US world, nobody should donate at higher levels at least

If they are a legitimate candidate. They get a set budget and must account for every penny.