John Banks urged the mother of his illegitimate child to have an abortion and then supplied drugs to make her miscarry and pressured her to take the necessary dose, court documents allege.

The woman, who says in a sworn affidavit she became pregnant after having sex with Banks in a Hamilton motel, claims she refused to take the pills and the two-term Auckland mayor ended their relationship.

She also claims he encouraged her to form a new relationship then pretend her next sexual partner was the child's real father. She subsequently did - perpetrating the lie for three decades, she says in a sworn affidavit.

The events are alleged to have occurred in 1969. But they played out today before the High Court at Auckland, where Banks faces a paternity claim by Japan-based English language teacher Antony Shaw.

Shaw, now 47, grew up believing his father was Chinese man Harry Wong, who was in a brief relationship with Pamela Mayes (nee Shaw) around the time she fell pregnant.

She told Wong he was Shaw's biological father and the Hamilton waiter paid her $5 a week in maintenance for the next 15 years.

But Shaw says he was always confused by his lack of Chinese features and was bullied at school, including being called a "chink".

He only learned that Banks could be his paternal blood relative when his mother broke the news to him by letter in 1999.

"Shocked" and "devastated" by the revelations, he admits being surprised by the likeness when he eventually saw a photo of the conservative politician and former Cabinet minister.

"I did look like him."

However, repeated attempts to confront Banks "face to face" proved fruitless, court documents say.

When Shaw finally tracked him down in an Auckland courthouse last year, Banks allegedly was reluctant to speak to him. An aide blocked Shaw's path then told police Shaw was "mad and stalking John", it's alleged.

The Herald was granted exclusive access to court documents relating to the paternity case, including sworn affidavits by Mayes and Shaw.

They allege Banks admitted to having fathered Shaw when he was a travelling salesman for Pfizer Pharmaceuticals and that he wanted to meet his illegitimate adult son.

But the documents allege he refused to undergo a DNA test on the advice of his QC lawyer, and wanted the situation kept "under wraps" due to his public profile and fears the story could be leaked to a woman's magazine or broadcaster Paul Holmes.

Mayes decided to finally "tell the truth" after Shaw's wife gave birth to the couple's only son in 1999 - allegedly making Banks a grandfather - and after Banks made a moving valedictory speech in Parliament referring to his own "crushing heartache" of growing up without a father.

"The number one problem confronting New Zealand today is the breakdown of the home and the family," Banks said as he bowed out of Parliament.

"We need to exalt mothers and uphold fathers. Children need mothers and fathers. A welfare cheque is not a husband. The state is not a father."

However, after a meeting to discuss Banks acknowledging Shaw, it's claimed Banks phoned Mayes and "went off his tree". He allegedly warned her not to take the matter any further, accused her of wanting money, labelled her mentally unwell and threatened to sue her, an affidavit says.

Banks has refused to engage with the court proceedings or respond to repeated requests for comment on the case.

He made no appearance at today's hearing and Justice Patricia Courtney reserved her decision on the paternity claim.

After today's hearing Shaw said through his lawyer that he was "extremely disappointed that he has had to endure an extremely stressful, costly and public court action that could have been avoided".

Shaw's lawyer, Jacque Lethbridge, said her client and his family had tried to speak to Banks on a number of occasions.

"My client hopes justice prevails," Lethbridge said.