The claims have left Ardern "deeply concerned and incredibly frustrated" because she was repeatedly told by the Labour Party that the complaints were not of a sexual nature.
Haworth has doubled down, saying that neither he nor the panel were told about the sexual assault allegation.
But the man said that is wrong as he raised claims of sexual assault with the panel.
"I told them about his experiences with other women that were tantamount to sexual assault ... but of course being aware that they weren't my stories and they would have to talk to those individuals themselves as part of that investigation process," he told Radio NZ.
He said that he also personally raised it with Haworth.
"Nigel Haworth knew that there were elements of sexual assault and sexual abuse within the claims brought before the investigation committee. That was very clear from the beginning. Everyone on the investigation panel knew that."
He said other complainants raised those issues with the panel, but those elements were missing from transcripts of their evidence - including his own - when they were given to the complainants.
"I discussed that there was a degree of sexual abuse and sexual assault by this individual ... Those things were discussed and on review of the transcript from my interview, that was not recorded."
Haworth has been contacted for further comment.
The man said that he challenged the staffer over his behaviour, and the response was an attempted punch that missed the mark.
On another occasion, he claimed the staffer shoved him into a wall.
He described the whole process as "deeply re-traumatising", leaving complainants with the feeling that justice would not be served.
Meanwhile Parliamentary Service general manager Rafael Gonzalez-Montero told the Herald that a complaint has been received about the Labour staffer that was related to the sexual assault allegation.
"It is not related to his employment with Parliamentary Service, so we can't do anything at this stage."
Ardern said the Dew review will provide clarity around who knew what and when, and if it finds failings from Haworth, then he should resign.
Dew is expected to report directly to Ardern within four weeks.