A convicted drug baron says police illegally intercepted phone calls he had with his lawyer, giving prosecutors unfair insights into his case.

Max John Beckham, a Northland businessman and farmer who led a double life as a druglord, has taken his case to the Supreme Court.

The Mangonui man is serving an 18 year jail term with a minimum non-parole period of nine years.

He was jailed for conspiracy to manufacture and supply methamphetamine and supplying meth, cocaine, cannabis oil and ecstasy.

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The Supreme Court was told today that Beckham was locked up after some 220 hours of phone calls were tapped. His car and cellphone were bugged and he was arrested in December 2008.

He was a high-value target and police spoke publicly of their relief at Beckham being taken "out of circulation" when he was jailed.

Beckham's lawyer Simon Mount told the Supreme Court that phone calls Beckham made to his then-lawyer Murray Gibson were intercepted, giving police insights into discussions that were legally privileged.

Mr Mount said it was "extraordinary" for senior police to authorise or tolerate surveillance of conversations that gave the prosecution a heads-up on the defence strategy.

"We've got material that is clearly subject to litigation privilege."

Originally, Beckham was sentenced to 13-and-a-half years' jail. He appealed that sentence but the Court of Appeal shot down the appeal and bumped up his sentence to 18 years.

The Supreme Court case relates to whether the Court of Appeal wrongly stated, and wrongly applied, the test for sentence reduction as a remedy for police misconduct.

That police misconduct, the appellant said, amounted to a breach of the Bill of Rights Act.

The bugged conversations included calls Beckham had with Mr Gibson when Beckham was a remand prisoner.

Mr Mount said the Department of Corrections already admitted to an "oversight" allowing privileged calls to be recorded.

He said police undertook "a large-scale data fishing exercise" to prosecute Beckham.

Mr Mount today said the prosecution gained a "tactical and practical advantage that our system simply does not allow".

He said search warrants granted for electronic surveillance were so flawed, all subsequent information was unlawfully obtained.

And he said some material was also unlawfully obtained because it was protected under lawyer-client privilege.

"The harm is that the balance of a fair trial, as it exists in our system, requires that the Crown is not forewarned," he added.

One such conversation that allegedly gave police a heads-up related to strategies Beckham planned on using to counter claims he had meth precursor chemicals.

It appeared police learned in advance Beckham would claim iodine was for his cows, acetone for paint, and caustic soda for cleaning drains.

Mr Mount said it was problematic this information was acquired ahead of trial, regardless of whether Beckham's excuses were valid or not.

"It's helpful to know where your enemy is weak as well as where your enemy is strong."

Mr Mount said police rode roughshod over a decades-old principle that mechanisms protecting solicitor-client privilege must be established. He said no such safeguards were installed in this case.

The Supreme Court justices appeared to voice scepticism at suggestions that an independent, unbiased person would need to monitor every call every prisoner made when in custody.

Justice Arnold asked Mr Mount whether this meant there had "to be a process in every one of these cases for vetting all the calls" prisoners made.

Mr Mount said yes, when it was known such calls would likely be privileged.

"In practical terms that means you'd have to pay people twice to listen to those calls. That really is, in practical terms, just about impossible," Justice Glazebrook said.

The court has adjourned for lunch and the Crown has not yet addressed the court. The hearing is expected to conclude tomorrow.