"Something needs to happen before someone is critically injured," said Ms Tuaine, a former Whanganui District Health Board member.
She was told a group of people went to the party later in the evening with the intention of causing havoc and her relatives were among those violently assaulted.
Mr Wainhouse said the late arrivals had connections with the Black Power gang.
Ms Tuaine guessed that some young people did not complain to police because they were afraid of retaliation from the gang.
Party-goers paid a fee to get in and ages ranged from 13 to 30. The over-18s could take their own alcohol to the party and there were also no age restrictions or ID checks.
Ms Tuaine said the parties seemed to go well apart from underage drinking, but "when people want to make trouble, that's the place to go to".
She was concerned at the level of violence at Polson St, and even more concerned that people were allowed to organise so-called private functions and admit paying guests. "I would encourage parents to view the photos [of the party, now out on social media] and understand where their youths are going and to make sure they have a safety plan."
She said no event organiser could keep 400 visitors, mainly young people, safe.
Mr Wainhouse said there had been three big, similar parties in the past four weeks. The previous ones were in Eastown Rd and Anzac Parade and he'd heard of entrance prices from $5 to $8.
The parties involved the same people, with similar numbers attending.
"Policing these events is problematic. The organisers tend to conceal the party location and advise party-goers a day or two prior to it being held via text and social media sites."
That was done to prevent police stopping the parties.
"Last week police inquiries established that the party held at Polson St was destined to be held at a rural Wanganui address. When we spoke to the owner of that address they were unaware of the extent of what had been planned and put a stop to it."
Police are investigating whether charges can be laid under the Sale of Liquor Act and would like to hear from others who were assaulted or who witnessed assaults. Anyone with information can ring Whanganui Police on 06 3490600, or call anonymously on 0800 555 111.