The mother of John Banks' illegitimate son wrote to the politician when she decided to come clean 27 years after the birth, telling Banks "you can only keep things buried for so long".

The handwritten letter was filed as evidence in Antony Shaw's successful paternity claim against Banks last year but its contents were suppressed.

A judge, who in August ruled the former Cabinet minister and Auckland mayor was Shaw's legal father, has also ruled the letter is now admissible, meaning its contents can finally be reported 18 years after it was penned.

The court accepted that Pamela Mayes had sex with Banks in a Hamilton motel in July 1969, when he was a travelling pharmaceuticals salesman.

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On learning of the pregnancy, Banks urged Mayes to have an abortion and supplied her with drugs to make her miscarry, urging her to take the necessary dose, the court heard.

When she refused, Banks told her to form a new relationship and tell the other man he was Shaw's father. She subsequently did, and Shaw grew up thinking he was half Chinese.

On November 23, 1999, Mayes wrote to Banks saying she was no longer prepared to lie. "I've kept to your rules for 30 years - you wanted me to have an abortion and even gave me quinine from the Drug Co you worked at.

"When I wouldn't take that you told me to tell someone else it was their baby.

"As much as I didn't want to I did, then had to live with the lie and parent our son by myself for four-and-a-half years - in the days before income support."

Mayes met with Banks twice that year to discuss coming clean after watching his emotional valedictory speech in Parliament, in which he discussed the pain of growing up without a father.

"I am prepared to face up to the past and the lies and take responsibility for the wrongness that I did also," she wrote.

"You really did have control over me in those days. I should say I allowed you to control me but then I would have done anything you wanted me to do (except the abortion - I couldn't do that!)"

She continued: "There is a lot of pain and hurt still for the way I was treated and let down by you. I'm feeling really stressed and tearful now that it has all resurfaced again.

"I guess you can only keep things buried for so long."

Mayes told Banks her marriage had been destroyed by the subterfuge and she feared she could also lose her son because of the lies they had perpetuated. Those fears were later borne out and she and her son are now estranged.

"I will have to live with that also," she wrote. "But I have weighed things up and even if Antony doesn't want to know you or me after this it is important to me that the truth be told and you acknowledge him on his birth certificate.

"It seems to me that you want it all to go away - it won't.

"You said nothing would change and you would not make a fuss - would keep it private with our family - not even tell your adopted son except maybe when he was older.

"I feel that you have missed out on having a relationship with your son for nearly 30 years. (Your choice).

"Now you have a grandson and I think you will miss out on having a relationship with him."

After receiving the letter, Banks warned Mayes not to take the matter any further, accused her of wanting money and threatened to sue her, the court heard.

After several failed attempts to meet Banks to discuss the paternity issue, Shaw eventually filed proceedings in the High Court.

Banks has refused to respond to questions about the case and took no part in the civil case.