You are not permitted to download, save or email this image. Visit image gallery to purchase the image.
In a statement to RNZ, the man said he had made the difficult decision to resign because of stress and to avoid being a distraction.
"I have enjoyed my time working in Parliament, but today have made the very difficult decision to resign because of the stress of the situation, and my wish not to be a distraction to the work of the government," he said.
"I adamantly refute the serious allegations made against me. I co-operated fully with the initial inquiry."
The staffer said he was now taking part in the new review of the whole complaint process and would continue to do so, "having been assured that this process will be fair to all parties".
Claims of bullying, harassment and sexual assault were reported about five weeks ago.
Labour had launched an internal investigation into seven formal complaints about the staffer's behaviour, and ultimately concluded that no disciplinary action was warranted.
That decision is now being reviewed by Maria Dew, QC, after the complainants approached media and National Party deputy leader Paula Bennett with concerns about the complaints process.
Prime Minister Jacinda Ardern has repeatedly stated that the man's employer - Parliamentary Service - was unable to take any action regarding the man as it had received no formal complaints from other staff members.
The man has been working away from Parliament's precinct for the past five weeks.