There is precedent for private texts sent to a Prime Minister to be made public, but a media law expert says whether it would apply to Jacinda Ardern's text from Richie Hardcore would depend on the context.

Hardcore, a friend of convicted Czech drug-smuggler Karel Sroubek, sent Ardern a text message after news broke about Sroubek being granted New Zealand residency.

Ardern said it was an unsolicited text message commending Immigration Minister Iain Lees-Galloway's original decision - which he has since overturned - and she did not reply to it, nor did Hardcore send her any other texts regarding Sroubek's case.

She said she did not say anything about the text previously because she did not consider it to be lobbying on Sroubek's behalf - and she had only been asked about that.

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National leader Simon Bridges has demanded Ardern release the text, describing her refusal as the Government's latest attempt to avoid scrutiny after initially refusing to discuss details of Sroubek's case, and then citing privacy reasons for not naming anyone who lobbied on Sroubek's behalf.

"The Prime Minister promised to be open and transparent but she is proving to be economical with the truth. This is the latest example of her dodging rightful scrutiny," Bridges said.

Chief Ombudsman Peter Boshier has previously ruled that the contents of an unsolicited text message to a Prime Minister could be released under the Official Information Act - depending on the context.

In 2015, then-Prime Minister Sir John Key received a text message from Rachel Glucina, who wrote a story in the Herald naming the waitress whose pony-tail Key had repeatedly pulled.

Like Ardern, Key did not respond to Glucina's text.

After an OIA request and complaint to the Ombudsman from blogger No Right Turn, Boshier said the content of the text should be made public.

"I am not satisfied in the circumstances of this case that good reason existed ... to withhold information in order to protect privacy interests or an obligation of confidence," Boshier said in his ruling.

"Even if an obligation of confidence existed, I consider it is outweighed by the high public interest in the information."

Canterbury University dean of law Ursula Cheer said the text from Hardcore to Ardern, if Boshier had to consider it, would also depend on the context.

"Clearly there is a precedent, but it would be highly contextual. It will depend on all the facts of this case. I don't know the facts to possibly predict what the Ombudsman would say."

As it was for the text to Key, Boshier would have to weigh public interest against factors such as privacy, Cheer said - but public interest did not mean the amount of media interest.

"It's a matter of immigration, of what a Minister and a Prime Minister did in relation to a high profile case, so there's certainly an argument of public interest there."

Cheer said if Boshier decided the contents of the text did not reflect Ardern's description of it as not lobbying, it might add to the case to release it.

But questioning the veracity of Ardern's description was not a valid reason to complain to the Ombudsman, she added.

The text to Key was different because it included the principle of a journalist wanting to protect a source - though the Press Council ruled Glucina had not disclosed her role properly, casting doubt about whether she was acting as a journalist.

There was also less of a privacy issue because Glucina had intended to publish the interview she was texting about.

Ardern said this morning that she might need to change her mobile number, as she had had the same number for a decade and received texts from thousands of people.

She told Newstalk ZB she was worried about setting a precedent if she released the content of Hardcore's text.

"People do [text] me all the time and people email me all the time and I just worry a little bit about precedent there, because members of the public email me, obviously, thinking that they will be doing that with some level of privacy."