The court testimony from Australian Competition and Consumer Commission (ACCC) enforcement director Michael Taylor on Monday points to a defense strategy questioning the integrity of an investigation that ended in charges of collusion during a A$2.5 billion ($1.67 billion) share sale in 2015.

In pre-trial hearings, lawyers for the investment banks and their client, retail lender Australia and New Zealand Banking Group Ltd (ANZ), have been trying to show witness statements by a third investment bank which worked on the deal, JPMorgan Chase & Co, were tainted by coordination between JPMorgan's lawyers and the authorities.

JPMorgan and its staff and ex-staff agreed to act as informants with the ACCC in exchange for immunity from prosecution, the court has previously heard.

On Monday, Taylor, the first ACCC employee to testify, was handed a note, and Deutsche lawyer Murugan Thangaraj told the packed courtroom it showed a JPMorgan lawyer notifying the ACCC about a discrepancy between statements from JPMorgan witnesses.

"That's what the file note records," Taylor said.

"That's an example of the JPMorgan lawyers attempting to give advice to the ACCC about how to build a case," said Thangaraj.

"That's your proposition," replied Taylor. "If there are inconsistencies, I want to bring them out. I want to know."

The JPMorgan witnesses had seemed to be "full, frank and truthful" throughout their ACCC interviews, Taylor added.

Taylor said he had taken charge of investigations for 30 years and that in this instance, the interviews demanded "more governance than I have ever been involved in".

More than a year and a half since the charges were brought against Citi, Deutsche, ANZ and several of their executives, the matter is yet to go to trial. None of the accused has entered a formal plea, but all have said they will plead not guilty.

The case hinges on allegations the banks colluded to keep hold of ANZ stock to prop up its price. It is being closely watched by investment bankers around the world because it could have implications about the way they are allowed to run share sales.

During a sometimes tense local court hearing, Taylor was presented with another email which Thangaraj said showed him arranging to meet JPMorgan lawyers without obtaining the necessary clearance.

When Taylor said he was unclear of the email's significance, Thangaraj said, "You are perfectly clear what this is about."

"I am absolutely not," said Taylor. "I don't know if the meeting occurred. I've read the email. I honestly don't know. I would not ever act with impropriety. I would never ever act inappropriately."

When Taylor said again that he was confused by the significance of the email, Thangaraj told the court, "that's rubbish."

The hearing continues.

By Byron Kaye