However Parker continued to host students at his house.
Most of Parker's offending occurred after the police warning.
Detective Senior Sergeant Kevin Burke told Radio New Zealand's Morning Report programme he had reviewed the file of the 2009 investigation, which he described as "thorough".
Mr Burke said police are bound by the Crown Law Prosecution Guidelines and the decision not to proceed with the investigation was based on those guidelines.
He did not agree the previous investigation had failed the victims.
"In hindsight, the investigation was carried out to its fullest and the correct authorities were informed," Mr Burke told Morning Report.
He said new cases had come to light and it is possible more charges could be laid.
"We still want to hear from any persons who may be able to assist the investigation and any potential victims out there, we'd urge them to contact on 0800 900 502."