The Western Australian premier, Roger Cook, has apologised after calling the US vice-president, JD Vance, a “knob” at a pre-election event in Perth.

At the West Australian’s Leadership Matters event on Tuesday morning, at which Cook was the guest speaker, a journalist asked the premier to finish the sentence: “JD Vance is a …” Cook replied: “Knob.”

Cook then said: “You’ve got to have one unprofessional moment, don’t you? That was it.”

His description won him laughs and a round of applause from the audience.