Trade union inquiry a 'witch hunt': Greens

The royal commission into trade unions is a "witch hunt" and highlights the need for a national anti-corruption body, according to the Greens.

Senator Scott Ludlam says governments setting up royal commissions into their political opponents is a misuse of power, and questioned why there isn't a federal corruption watchdog.

"Royal commissions are among the most powerful, high-powerful investigative mechanism we have in this country - now three opposition prime ministers have appeared in front of them," the acting Greens leader said.

"There's no question that it's a witch hunt."