The Russian ambassador can't be summoned to Parliament for questioning as he has diplomatic immunity, the Foreign Affairs Minister has told MPs.

RNZ revealed this week the Foreign Affairs Select Committee is investigating whether it can summons Georgii Zuev, after he twice rebuffed their requests to appear before it.

It is specifically looking into the issue of diplomatic immunity - under the Vienna convention the ambassador is not obliged to give evidence as a witness.

Foreign Affairs Minister Nanaia Mahuta said she asked for advice after hearing about the select committee's discussions.

Mahuta then sent a letter to the select committee this morning, which outlined the legal advice she had received on the matter.

"That legal advice showed that the ambassador was covered by the Vienna Convention and there was a level of diplomatic immunity that was afforded to him."

It may not be helpful for MPs to question him, Mahuta said.

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"Whether or not investigating or asking questions of him will give the information that the committee is seeking I think is something that is very arguable."

Ultimately the business of Parliament is a matter for the Speaker, she said.

The Foreign Affairs Select Committee does not have the power to summons a person.

It would have to apply in writing to the Speaker, who would then decide if the person should be sent for.

Earlier this week, Prime Minister Jacinda Ardern urged MPs on the select committee to heed the legal advice.

"I imagine they will be seeking legal advice over the legality of bringing in someone who essentially has diplomatic immunity ... my intent is to simply let the committee go through its own processes.

"The most important point for me to make is this is not the thing that is going to make the most tangible difference to those who are subject to this war."

Economic sanctions and contributing to the war effort will make the most difference, Ardern said.