Serial private prosecutor Graham McCready has threatened to take the Todd Barclay recording case to court if police don't immediately reopen their investigation.

"It appears that prima facie an offence of unlawfully recording of conversations took place and that Prime Minister Bill English is an accessory after the fact," Mr McCready, who once successfully litigated against Labour MP Trevor Mallard, said in a statement.

But legal expert Andrew Geddis told Newshub it's unlikely Mr McCready would be successful in his prosecution or that police will reopen the investigation due to a lack of solid evidence.

Clutha-Southland MP Todd Barclay announced last week he'd step down from politics once his current term ends, after it was revealed he'd told Prime Minister Bill English he recorded Glenys Dickson in his Gore electorate office. Mr English made a statement to police in April last year and said Mr Barclay told him about the recording. Police closed the investigation into Mr Barclay after ten months, during which he refused to speak to them and denied the recording allegations.

Mr McCready said "it is in the public interest" to file charging documents against Mr Barclay, but it's "conditional on the complainant and other witnesses giving sworn statements" to the court to show the trial is warranted. He'd name Mr Barclay as "the alleged principal offender" and Mr English as "either an alleged accessory or as a person who attempted to or did obstruct justice".

Mr McCready has a long history of filing private litigations. He made an unsuccessful attempt over John Key's pony-tail gate saga in 2015, and in 2007 when Labour party MP Trevor Mallard was convicted of assaulting National MP Tau Henare in Parliament.

'I don't think it'll work' - expert

Otago University law professor Andrew Geddis is sceptical about Mr McCready's chances of success in prosecuting Mr Barclay.

"I don't think it'll work because he'll run into the same problem the police have run into. While we may in the court of public opinion draw certain conclusions about Todd Barclay's behaviour and what Bill English said about it and so on, those conclusions aren't the same as legal evidence that can establish guilt in criminal proceedings."

"I would be extraordinary surprised, based on what we know now, if the court allowed it to proceed."

Mr Geddis doubts that police will reopen the investigation into Mr Barclay, unless new evidence emerges or Mr Barclay decides to speak to police.

"What's really needed to reopen the investigation is new information and basically everything we've been finding out has come from the police files ... if police were to go back and reopen the investigation based on what we've heard it would raise real questions about why they didn't proceed in the first place. So I doubt it."

He said that the case was unlikely to be reopened unless they can get Barclay to effectively say "yeah I did it", or unless a tape emerges that can be tied back to Barclay.

Mr Barclay and Mr English have been contacted for comment.

Newshub.