Two conservative panellists have reneged on their promise to appear on this week’s ABC Q&A panel, with a Liberal cabinet minister also declining to act as a replacement.

On Monday afternoon, only hours before the episode was due to be recorded and aired, scheduled panellist Nick Carter, executive director of the Menzies Research Centre, cancelled his appearance.

This came after Tony Abbott’s parliamentary secretary Alan Tudge withdrew on the weekend.

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Both cited the controversial Zaky Mallah episode as the reason for their decision not to participate.

Communications Minister Malcolm Turnbull, who chided the broadcaster for “playing at tabloid journalism” by allowing the convicted criminal and acquitted terrorism suspect to ask a question on last week’s episode, also declined to act as a stand-in.

Mr Turnbull denied he was boycotting the show, telling journalists there was “no doubt” he and others in government would appear on the ABC in future.

Human Rights Commissioner Tim Wilson remains the lone conservative on the panel.

“I feel an obligation to go on Q&A and explain what free speech actually means,” Mr Wilson said.

Without replacements, the show’s panel will consist Mr Wilson, counter terrorism expert Dr Anna Aly, Labor’s Tanya Plibersek and physicist Lawrence Krauss.

The show’s topic will be freedom of speech.

Conservative commentators frequently criticise the ABC for a perceived left-wing bias, lampooning it as ‘their ABC’.