Australian Competition and Consumer Commission (ACCC) enforcement director Michael Taylor should not have sent draft witness statements to his personal email account and saved sensitive documents on USB sticks so he could work on the investigation at home, said Leah Won, a senior staffer at the regulator.

The disclosure in a pre-trial hearing underscores a key line of defence against the country's biggest white collar criminal case: the investment banks want to show the evidence used to charge them was tainted by departure from due process.

The banks and their former staffers are charged with colluding during a A$2.5 billion ($1.8 billion) 2015 stock issue to withhold unsold shares and keep the stock from falling. Their client, Australia and New Zealand Banking Group Ltd, is also defending the case.

The matter is being closely watched by financial markets participants around the world because it could influence how capital raisings are conducted.

Won, the ACCC's general manager of financial services competition, was asked if Taylor had acted within the regulator's own guidelines when he sent draft witness statements to his personal email address to review at home.

"No, I don't think that should have occurred," Won told the court. "It's a secure network and your home internet is not secure."

Accessing investigation documents stored on a USB stick through a home computer would also be a departure from ACCC guidelines, Won said.

The ACCC declined to comment. In previous hearings, Taylor has denied departing from due process.

The hearing continues.

(Reporting by Byron Kaye; Editing by Lincoln Feast.)

By Byron Kaye