The ABC and Fairfax Media have launched a court bid to overturn a Federal Court ruling throwing out their truth defence in a defamation case brought by Chinese-Australian businessman Chau Chak Wing, saying the ruling works a "substantial injustice" and deprives them of a key plank of their defence.

Mr Chau, one of Australia's most generous political donors, filed defamation proceedings in June last year against the national broadcaster, the newspaper group and Fairfax journalist Nick McKenzie over a joint investigation including an ABC Four Corners program and online article.

Chau Chak Wing leaves the Federal Court in June, earlier this year. Credit:Peter Rae

His barrister, Bruce McClintock, SC, has told the Federal Court the publications paint Mr Chau as a spy who "betrayed his country" in order to serve the interests of China and the Chinese Communist Party.

The media outlets and McKenzie were relying on a range of defences but a central part of their case was that the claims were substantially true.