University of Otago law professor Andrew Geddis made the comments after police announced yesterday they were reopening the investigation into the allegation the Clutha-Southland MP had secretly recorded his staff.
Allegations also surfaced yesterday the secret recordings made in Mr Barclay's office included discussion about ''sex and drugs''.
Prof Geddis said it would remain difficult for police to prosecute the MP even if they had access to the tapes.
''It's not enough that you just have tapes of a conversation; you have to show they were intentionally made of a conversation that was private.
''Those are elements that need to be proved beyond a reasonable doubt, so it's difficult.''
But the reopening of the investigation could definitely be damaging for the National Party in an election year.
''Mr English's statements on this have basically been it was investigated for 10 months and the police were satisfied.
''That no longer appears to be true, and that allows the story to continue to run despite Mr English's efforts to put it behind him.
The big question was whether Mr Barclay would co-operate with police this time.
The police investigation file released to the Otago Daily Times under the Official Information Act in March revealed Mr Barclay declined during the initial investigation despite earlier telling the ODT he would fully co-operate.
Prof Geddis believed Mr Barclay would once again refuse to co-operate, given his political career appeared to have ended.
''I suspect he will take legal advice and decline. Put it this way, if I was Mr Barclay's lawyer I would be strongly advising him not to speak.''
He was surprised that police had reopened the investigation.
''I was on record as saying I didn't think they would reopen it, because I wasn't sure what new evidence had come out in the last few days.
''It seemed to me that everything being reported was coming from essentially the police files themselves or from individuals who had already been interviewed by police.
The only new information to police he could think of was Mr Barclay saying he agreed with Mr English's statement to police.
''Maybe police are taking that as an indication that Mr English's version of events has been somewhat confirmed by Mr Barclay.''
Rather than the emergence of any new information, it was likely police were wanting to make sure the investigation was ''not just comprehensive but is seen to be comprehensive''.
Mr Barclay did not respond to questions from the ODT, including one asking whether he would co-operate with police.
Mrs Dickson also declined to comment on yesterday's developments and refused to confirm or deny a report she was re-interviewed by police in Invercargill on Monday.
Meanwhile, according to a report by Newsroom yesterday, Parliamentary Services knew what was in the tapes and that they contained discussions about ''sex and drugs''.
Newsroom quoted from Glenys Dickson's unredacted police statement from the original investigation.
Mrs Dickson, who was an electorate agent in Mr Barclay's Gore office, laid the original police complaint against Mr Barclay, though no charges were laid.
In the statement, Mrs Dickson said Parliamentary Services had acknowledged to her Christchurch lawyer Kathryn Dalziel there were recorded conversations involving her ''which concerned Todd Barclay regarding sex and drug matters''.
Mrs Dickson could only recall one conservation on this subject, with another National staffer in Queenstown.
It came after Mr Barclay pulled out of a prizegiving ceremony at the Telford Agriculture College in Balclutha.
Mr Barclay had been invited as the guest of honour but instead stayed in Queenstown after a big night out.
-Additional reporting by NZME
