By the time officers arrived the uninvited guests had left, but police forced the others out of the house.
IPCA chairman Sir David Carruthers said on the balance of probabilities, Mr Christie, suffered a broken neck when he was hit with a baton by a member of the Tactical Policing Unit.
"This force was excessive and contrary to the law," Sir David said
The report was not able to identify the responsible officer.
"Although some of the force may have been used in self-defence and therefore justified, other force was used for the purpose of unlawfully removing partygoers from the house and was therefore unlawful."
Mr Christie today said he would fight for compensation for the police's "thuggery'' which left him with continuing physical and emotional problems.
Since the party the 23-year-old's confidence had taken a blow and he now found himself withdrawing during gatherings when he used to be outgoing.
His lawyer Keith Jefferies said he would be seeking a "significant'' amount of compensation, but would not be drawn on a ballpark figure.
The physical and emotional harm to Mr Christie would have to be taken into account, he said.
After the party, doctors said Mr Christie had been close to being paralysed, Mr Jefferies said.
"He still suffers from depression from time to time.''
Mr Christie said that every morning since the party, his necks clicked and during cold weather his back ached.
The IPCA report went a "long way'' to clear his name, but there would always be people who would believe the police version of the event, he said.
Iain Morrison, who represented some of the partygoers, said there was a "culture of corruption'' in the police force.
"There's no excuse for ignorance of the law''.
He described the actions of the officer who hit Mr Christie from behind with the baton as a "pretty cowardly act''.
Police also failed to conduct a timely, thorough and robust report which was "unjustified, unreasonable and unfair" to the complainants and police officers involved, Sir David said.
"Such a delay is inexcusable."
He acknowledged the authority also contributed to the delay in releasing the report, including waiting for the police to conclude its own investigation.
"The authority has recently changed its processes to address this issue and ensure that it completes investigations more quickly than it has done in the past."
Police need to review the way they conduct matters on private property and they have begun procedures to change their policy, Sir David said.
A subsidiary complaint to the IPCA regarded the leak of a police preliminary report into the incident to a media outlet, by an office administrator.
The disclosure of the report was an attempt to discredit a complainant, Sir David said.
"As the administrator had left the police when this matter was discovered, no further action was taken by police."
A second subsidiary complaint alleged three Tactical Policing Unit officers had made false declarations about the events at the party, but the IPCA found the three officers involved in shutting down the party did not deliberately lie or make false declarations.
An internal investigation of police behaviour at the party cleared officers of any wrongdoing, despite 10 complaints of police using excessive force and being overly aggressive.
Today, police accepted the two specific recommendations made by the IPCA in relation to how police dealt with disorder situations on private property.
Police said they also accepted the IPCA's conclusion that the Tactical Policing Unit's decision to enter the property against the wishes of the occupants was contrary to law in this instance.
"In addition we accept the possibility that the neck injury suffered by [Mr Christie] could have been caused by a police baton while the party was being shut down, however, the police investigation into this specific allegation was unable to identify sufficient evidence that met the threshold for initiating criminal proceedings," police said in a statement.
Police regretted that several weeks after the party, a police employee leaked information to the media in relation to their investigation into the allegations.
"Police have apologised to [Mr Christie] for breaching his privacy and acknowledge the distress this action caused.
Police also accepted that extensive delays occurred during their internal investigation into the incident and it was regrettable it had taken four years for this matter to reach this point.
"We acknowledge that not enough resources were initially allocated to the case to ensure a robust and timely investigation was conducted."