The head of one of New Zealand's top spy agencies says a blanket ban on all foreign donations would not be an effective way to curb potential foreign influence in New Zealand's elections.

But improving disclosure requirements around donations would help improve New Zealand's intelligence agencies' ability to trace donations.

"This would assist our investigative work," New Zealand Security Intelligence Service (NZSIS) boss Rebecca Kitteridge told MPs this afternoon.

Kitteridge and Government Communications Security Bureau (GCSB) boss Andrew Hampton addressed the Justice Select Committee this afternoon as part of the Government's inquiry into the 2017 election.

The pair had already submitted to the committee in April, but were invited back to talk about a few more issues today.

One of these issues was around donations being used to influence and leverage MPs and political parties.

This comes after questions continue to be raised about a $150,000 donation to the National Party from a Chinese-owned, but New Zealand based, company.

Speaking to media before addressing MPs, neither Kitteridge nor Hampton would address this specific example.

But Kitteridge did say NZSIS were aware of some political donations which have been made that "have not been completely transparent".

She would not, however, go into detail about these donations but later told MPs this activity "spans the political spectrum and occurs at a central and local Government level".

• Andrew Little: Foreign donation ban a possibility

She also talked to MPs about the issue of donations when it comes to political influence.

"From the intelligence we have, we know that a foreign donation ban, would not, on its own, be an effective way of mitigating the risks New Zealand candidates and MPs face."

She said it would not be hard for a foreign entity to work around such a ban, if it was in place.

"We know that foreign states are adept at understanding and working around regulatory regimes."

The Greens this morning renewed its calls to ban foreign donations to political parties in New Zealand.

Meanwhile, Justice Minister Andrew Little told Parliament today that he would prefer to wait for the justice committee to report back on its inquiry into the 2017 election before taking action on vulnerabilities in the law before the 2020 election.

But he also indicated that he would not wait forever.

"There will come a point at which I will need to consider whether the Government acts before the justice committee provides its report and its recommendations," Little said.

"It would not be my preference to do so before having the benefit of that committee's collective wisdom but I will closely watch the progress that the committee makes on its report on the inquiry into the 2017 election."

He was responding to a question from former National MP Jami-lee Ross who wanted to know why the minister could not address the issue of foreign donations before the 2020 election.

Little said he admired Ross for having consistently expressed concern about the issue, to which National MP Nick Smith interjected: "not consistently."

Little was also questioned by Green MP Gholriz Ghahraman about the Herald story on the $150,000 donation via a New Zealand-registered foreign-owned company and he said the law appeared to have a vulnerability.

"If it is correct that the source of that revenue for that company is foreign, that the source of the capital for the company is foreign, then that would appear to have defeated the objectives of the law that we have. That's why I say our law appears to contain a vulnerability."