A former longtime MP says an investigation's needed into the claims made by Gaurav Sharma.

Peter Dunne says as soon as the rogue MP levelled accusations against Labour, its whips and the Prime Minister's office Jacinda Ardern should have commissioned a probe.

Sharma was expelled from the caucus on Tuesday and faces expulsion when caucus meets again in four days.

But Dunne says told Heather du Plessis-Allan it might be too late for an independent investigation into the allegations.

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