The Port of Brisbane has defended a decision to ban a security guard over an anti-coal industry sign on her car, arguing it would stop her doing her job properly, court documents show.

Kym Garrick's employer Corporate Protection Australia Group fired her from her job at the port in August last year because she displayed a sign that read "coal dust free Brisbane".

Her lawyers alleged the Port of Brisbane and her employer discriminated against her on the basis of her political beliefs.

In court documents obtained by the ABC, the Port of Brisbane said it was "lawful and perfectly reasonable" to ask Ms Garrick to remove the sign.

The documents stated that if Ms Garrick was allowed to keep the sign in her car, it would "compromise that employee's ability to deliver effective security services" and "cause reputational damage to the Port of Brisbane".

Ms Garrick's lawyer Giri Sivaraman said that argument did not stack up.

"We think the greater detriment to reputation might be the way they treated Kym - the unlawful and very harsh way they treated her," he said.

"There's no evidence to show there was any damage to reputation or any effect on stakeholders.

"It's a small sign, inside her own personal vehicle that she's driving to the site."

In the court documents, the Port of Brisbane argued the request to take the sign down was "all the more reasonable" because Ms Garrick could be monitoring or controlling protesters at the port.

It said it was concerned Ms Garrick would be "unable" or "disinclined" to do her job if protesters tried to access the coal stockpile at the port.

Mr Sivaraman said there was no evidence to back that up.

"She was good at her job - she had a good performance review," he said.

"There's never been a protest at that site while she's been there.

"It's all just been raised after the fact to try and justify what we say was an unlawful decision."

Corporate Protection Australia Group and the Port of Brisbane declined to comment on Monday.

The case is due in court on Tuesday.