The lawyer of the former Malaysian diplomat who indecently assaulted a Wellington woman in her home has told the court his client's behaviour was deteriorating in the weeks leading up to the offending because he was suffering from a mental illness.

Muhammad Rizalman bin Ismail gave evidence during a disputed facts hearing in the High Court at Wellington last week after changing his plea to guilty.

The victim, Tania Billingsley, has not given evidence.

The court heard how Rizalman told two police officers after the indecent assault that "he'd lost his mind".

Defence lawyer Donald Stevens QC told the court today that Rizalman's colleagues had noticed him falling asleep at work.

He also said that some days Rizalman wouldn't show up to work, he'd be found wandering the streets not knowing where he was, and that he wouldn't interact with his wife and children.

"He was in a perpetual daze," Dr Stevens told the court in his closing submission.