Two mothers at one of Sydney's top private schools are suing each other for defamation after posting comments about each other on the instant messaging app WeChat.

Ava Wei Wu and Michelle Li Chen both had sons at Knox Grammar, in Sydney's northern suburbs, and became friends in mid-2011.

However, their friendship soured in March 2016 and both women took to various Knox parents' groups on WeChat, sparking a feud that has gone all the way to the NSW Supreme Court.

Ms Wu, a mother of two, claimed Ms Chen's comments convey the defamatory imputations that Ms Wu taught children to act with violence, attempted to commit murder and tried to run over a policeman.

In a statement of claim, obtained by ABC News, Ms Wu said her former friend made her out to be a "savage person", an incompetent manager and an unethical person in a series of posts on March 5, 2016. She claims some of the posts were seen by up to 1,000 WeChat forum members.

The two mothers had sons at prestigious private Sydney school Knox Grammar. ( Google Maps )

However Ms Chen launched her own defamation case, claiming Ms Wu published a series of offensive posts about her.

Ms Chen said those posts were seen by at least 64 members of the English as a Second Language (ESL) group at Knox and 24 members of the Knox Mothers WeChat group.

Mothers trade barbs

Screenshots of the chats tendered to the court show Ms Wu wrote posts including: "In front of me you call me baby and darling, and behind my back you stab me madly with a knife.

"You really think that you are a high class lady by sending your children to a private school, and by buying a few fake handbags?

"And don't forget, the reason why your son got into Knox is because I noticed the vacancy for you and told you and acted as your referee, I even went to the office with you.

"Don't get so innocent when you are really a bitch! Let me see who is phony and pretentious! Fake Face, Michelle Chen! Look here!"

Ms Chen replied: "@Ava you vixen! Yeah? I just finished my meeting and I see you bitching like a vixen!"

The Knox Grammar mothers traded barbs in WeChat groups including Knox Grammar Mothers ( ABC News )

On separate forums Ms Chen accused Ms Wu of "destroying" another woman's family by having an affair with a married man in their native China.

"...such a person who was a mistress and destroyed the families of others is bad in essence," Ms Chen wrote.

"She was the one that boasted about helping my son get into private school by her acting as a referee, when I actually already had two friends write reference letters."

She also wrote: "How dare you attack others when you buy fake products yourself?"

And, "Ava, self-proclaimed chair of ESL, brutally attacked and abused me in our year 4 group and her friendship circle".



Ms Chen looks to truth defence

In her defence filed with the court, Ms Chen denied her posts convey the defamatory imputations Ms Wu claims.

However, if the court does find the imputations are conveyed, Ms Chen said she can successfully defend her conduct because what she had said was largely true.

She said on a 2012 playdate, Ms Wu encouraged Ms Chen's 12-year-old daughter to hit Ms Wu's 7-year-old. She further claimed at a dinner party Ms Wu's husband said his wife had once tried to avoid a random breath test and attempted to run down a police officer. And she accused Ms Wu of being an ineffective convenor of the Knox non-English-speaking parents group because of her limited grasp of English.

In May the mothers were ordered into mediation however they were unable to settle the matter. Earlier this month the case was set down for a five-day trial before Justice Lucy McCallum without a jury in July next year.

In a statement, Knox Grammar said: "The dispute is a matter between the two individuals and it would be inappropriate for us to comment further while the matter is before the courts.

"The social media group was not an official communication channel nor endorsed by the school."