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Ms Bennett told Parliament yesterday afternoon the man at the centre of the allegations worked in the Labour leader's office at Parliament.
Her comments followed the earlier resignation of Labour Party president Nigel Haworth.
''These are serious allegations. The Prime Minister cannot keep her head in the sand and pretend like it is happening somewhere far, far away; it is happening in her own office, in her own organisation - she is the leader of the Labour Party.''
She named Ms Ardern's former chief of staff Mike Munro, her chief press secretary Andrew Campbell and the director of her leader's office, Rob Salmond, as knowing about the allegations.
''I have been told by two victims who work in Parliament that they went to Rob Salmond around Christmas time and made a complaint about the alleged perpetrator.
''The Prime Minister has constantly said her office did not receive complaints and in fact encouraged victims to speak to their line managers - they did.''
Ms Bennett said it ''smacks of a cover-up'', and went ''straight to the top to Ms Ardern, to senior Cabinet ministers and to senior staff in her office''.
RNZ asked Mr Salmond to comment on Ms Bennett's claim.
In response, Ms Ardern's office issued a statement saying, ''we have no comment to make on the allegations made under the protection of parliamentary privilege''.
Finance Minister Grant Robertson, also named by Ms Bennett in Parliament, has refused to answer questions about whether he was told directly of allegations of sexual assault.
He was asked if a complainant told him about such an allegation in late June.
''I'm going to respect the privacy of the young people who are involved in this situation.
''There is a process to go through ...
''It's important the voices of these people are heard and I'm not going to say anything more about that.''