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, but the Court of Appeal bumped it up to 18 years. The Supreme Court case related to whether the Court of Appeal wrongly stated and applied the test for sentence reduction as a remedy for police misconduct. That misconduct, the appellant said, breached the Bill of Rights Act.
The bugged conversations included calls when Beckham was a remand prisoner. Mr Mount said Corrections had admitted to an "oversight" allowing privileged calls to be recorded. He said police undertook "a large-scale data fishing exercise" and the prosecution had gained a tactical and practical advantage.
Search warrants for electronic surveillance were so flawed that all subsequent information was unlawfully obtained, and some material also had been unlawfully obtained because it was protected under lawyer-client privilege, he said.
"The harm is that the balance of a fair trial, as it exists in our system, requires that the Crown is not forewarned." .
One conversation that allegedly gave police a heads-up related to strategies to counter claims Beckham had meth precursor chemicals.
Mr Mount said police rode roughshod over a decades-old principle that mechanisms protecting solicitor-client privilege must be established.
The Supreme Court Justices appeared to voice scepticism at suggestions an independent, unbiased person would need to monitor every call every prisoner made when in custody.
Justice Arnold asked if 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, if it was known such calls would probably 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 hearing is expected to conclude today.