The letter, signed by all six Greens MLAs, said Mr Morrison's response to the allegations raised concerns about the standard of behaviour expected by cabinet members.

"When these recent events are combined with other allegations coming from Parliament House it is reasonable for the public to have a lack of confidence in the ethics of their elected representatives," the letter said.

"Elected representatives must set a higher standard."

The letter criticised Mr Morrison's handling of the allegations, and said his conduct did not assure victims of sexual assault that they would be treated with respect when speaking out about abuse or that ministers of the crown would be held accountable for their actions.

"This matter cannot be simply brushed off with you saying you accept the word of the accused," the letter said.

"You have openly stated that you have not read the details of this case and appear to disregard the real and terrible impact on both the direct victim and other survivors of sexual assault.

"These events are highly distressing, most of all for women who are survivors of sexual assault.

"While it is distressing to learn of more alleged sexual assaults it is vitally important that survivors are encouraged and supported to seek justice."

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The letter said it was common practice within workplaces to have independent investigations into the conduct of employees and people with positions of power.

It also said that the High Court of Australia had set an example for such independent investigations "recognising that the highest legal offices of our country must be fully accountable and demonstrate that no office bearer is above scrutiny".

"We share your commitment to the rule of law," the letter said.

"We also submit that there are times, particularly when it comes to allegations of sexual assault, the criminal justice system does not deliver justice to survivors or to alleged perpetrators."

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