Vice-president of Young Act, Ali Gammeter, says in a tweet that she has resigned, claiming she has been a victim of sexual harassment.
"Tonight I resigned as Vice President of Young Act," she said.
"For months I have been sexually harassed, slutshamed, and ignored."
Gammeter, who is from Victoria University, said in the tweet she was "not the only victim of this behaviour in our ranks".
Act Party president Tim Jago said the party was taking her complaint seriously.
"We are taking these allegations very seriously. Any form of harassment is completely unacceptable. We will be providing Ms Gammeter with any support she requires," Jago said.
"Act is now considering a range of options, including appointing an independent person to investigate this matter."
Young Act said this morning "prevalent and systematic incidents of harm have occurred within its organisation".
Also, there was justified criticisms about how members and those using its platforms interact with each other.
"We have failed a standard of care that we owe our members, and for that we apologise," it said.
"We can and will do better."
The organisation said it will be investigating and following up on the reports of sexism and harassment thoroughly.
"Those responsible will be held accountable within the organisation itself and higher if that is the action victims want to go ahead," it said.
"We have taken steps to begin this investigation immediately through the monitoring of posted content that crosses the boundaries of reasonable discourse and our action to prevent future incidents of harassment from occurring within Young Act begins today."
Young Act said it would be producing an official equity policy.
"This equity policy will give us a framework to deal with bullying, harassment and discrimination and set a standard of behaviour required of members and those using our platforms that has been absent. This not only includes behaviour in the real world but Young Act online spaces as well."
President of Young Act Felix Poole said the youth wing had failed Gammeter and that they did not have a system in place to deal with complaints.
He said the organisation has started establishing guidelines to deal with harassment complaints in the future.
ACT Party leader and MP for Epsom David Seymour would not comment, but said the party will be issuing a statement later today.
Gammeter has also been approached for further comments.
- More to come