Without boulders rolling down the hill there was a perception that there was no issue.
"But if you have a boulder rolling down the hill tomorrow and someone's killed ... the criticism of council would then be 'you didn't do enough'.
"I think at the moment, the balance is right. We've issued the notices, the people can't say they were not aware and living in the house."
Mr Mitchell said the next step was to take civil court action seeking injunctions ordering people to leave.
The private investigator had been hired to make observations and gather information from other residents in order to provide evidence of occupancy, which was difficult to prove because people were still allowed temporary access to their properties.
The investigator's affidavit would then be presented to the court, along with the geotechnical advice.
"Then it's up to the court to decide what it wants to do."
Police would not be showing up to homes to evict people at 4am and the council has ruled out criminal charges, Mr Mitchell said.
He denied the use of an investigator was a step too far, saying: "At the end of the day I would use a council officer who would do exactly the same thing. So to me the label is not the important thing - the important thing is why we're trying to do this."
Mr Mitchell said the investigator was a former police officer who was registered with the Ministry of Justice.
"I don't have any concerns about the inappropriateness of anything he might be doing from that point of view," he said.
"He knows clearly what the rules of his game are, and from my point of view I'm quite confident, given the council is a public body and given this is public money that's being spent, that he's an appropriate person to spend it on."
Courts' closure "knee-jerk reaction''
Meanwhile, the Government's decision to close a number of courts because of earthquake risk is being described by Labour as a "knee-jerk reaction''.
The Ministry of Justice last night announced the closure of six of its district court houses in response to testing that followed the Christchurch earthquakes.
District court hearings in Upper Hutt, Masterton, Feilding, Rangiora, Oamaru and Balclutha were stopped from this morning, and the ministry has said they could remain closed for up to a year.
Labour's Justice Spokesperson Charles Chauvel described the decision as a "knee-jerk reaction'' and said a proper risk assessment should apply.
"These buildings must have been considered safe last week, and the week before. The acting deputy secretary of courts says there is `no imminent danger','' Mr Chauvel said.
"The courtrooms in these courthouses will be scheduled for skeleton sittings over those three or four weeks. The likelihood of a major quake in most of the affected locations between now and Christmas is low.''
Mr Chauvel said the courts' closure meant victims of crime might have to wait longer for their cases to be heard, and thousands would be forced to travel elsewhere for relocated hearings.
"Either the courts should reopen immediately and priority remedial work scheduled over the December/January period, or temporary courts need to be opened in risk locations, now.''
