The message was only 27 words long and visible online to a small audience for 38 minutes, but orphanage patron Geraldine Cox is suing Canberra law student Juanita Zankin for the harm she alleges the post caused to her reputation in Australia and abroad.

In defence, Ms Zankin argues she had merely tried to advise potential donors to inform themselves about allegations of degrading children in advertising and international opposition to orphanages.

The case against Ms Zankin - filed in the ACT Magistrates Court - stems from an exchange on Facebook in February.

Canberra law firm, Ben Aulich and Associates, posted an image of proprietor Ben Aulich with Ms Cox and a third person on its Facebook page.

An accompanying caption said: "An excellent time raising funds for Sunrise Cambodia with Geraldine Cox".

In response, Ms Zankin made a comment on the post: "Are you not aware of the harm this woman has caused? A shameful cause to raise funds for. This woman is despised in Cambodia. Do some research."

The lawsuit claims the post was defamatory and meant to cause harm to the operation of Cambodia Sunrise - the charity Ms Cox founded in 1997- in causing the former diplomat to be despised, to prevent Australians from making donations, and inferring that the orphanage operator conducts the charity shamefully.

Ms Cox asked the court to grant her damages, aggravated damages, and costs.

"The plaintiff claims aggravated damages by reason of ... her knowledge of the falsity of the imputations, and her hurt and distress at the failure or refusal of [Ms Zankin] to publish an apology and retraction," the lawsuit said.

But Ms Zankin argues the post was not defamatory, and even if it was, the comments were justified, protected by qualified privilege, represented fair comment, was an honest opinion, were in the public interest, and free speech.

In her defence, Ms Zankin admitted posting the message but said the law firm had invited comment and allowed it to remain on the site for 38 minutes before it was finally removed.

In a twist, Ms Zankin joined Ben Aulich and Associates as party to the proceedings, arguing it should also be liable for damages as the comment had been published on its Facebook page.

Sister firm, Aulich Civil Law, is representing Ms Cox, which the defendant claims is a conflict of interest since a related Aulich company is party to the proceedings.

Ms Zankin, in an affidavit filed to the court, said she had visited Cambodia in 2016 as part of a human trafficking unit at university.

She argued she had posted the comment "on the basis of what I had learned on the university study tour to Cambodia with the hope of encouraging potential donors to fully inform themselves before making a donation".

"A simple internet search shows that most organisations including The Cambodian Ministry of Social Affairs, UNICEF, and Save the Children oppose the orphanage model and advise donors to redirect support to organisations working to keep families together."

Ms Zankin said other concerns raised on her visit included, that Ms Cox had "disclosed masturbating a disabled child to 'get her to sleep'", that 75 per cent of children in care in Cambodia had parents, that tourists are encouraged to visit orphanages which put children at risk of physical and psychological harm, and that Sunrise Cambodia had "advertised in such a way that degraded children" in contravention of codes issues by the Australia regulator.

"I did not want to raise the specific concerns on the post but instead wished to raise awareness for the issue."

The defamation case looks destined to be determined in court after Ms Cox took issue with Ms Zankin's pleadings.

The case appeared before the registrar last month to hear a number of pre-trial matters, including an attempt to strike out the defence, a bid to have the lawsuit dismissed as vexatious, and an application for security for costs.

The decision was reserved and is scheduled to be handed down later this month.