A police forensic technician looks for evidence at the scene of a rape in Sockburn, Christchurch.

Police are warning a rapist posing as a contractor has been knocking on doors in a Christchurch suburb.

About 30 police officers were scouring the city for the man who police say attacked a young woman in her Chrischurch home.

Officials described the offender as a Polynesian in his 40s with two front teeth missing, and said he wore a high-visibility vest when he went to the woman's Sockburn home shortly before 10.50am yesterday.

It was likely the man door-knocked other homes in the Sockburn area in the days and weeks before the attack, said Detective Senior Sergeant Darryl Sweeney, who is heading the investigation.

"We would like to hear from anyone in the area who has noticed any suspicious activity or vehicles in the vicinity over recent days and weeks, including unfamiliar people knocking on doors or visiting properties," he said.

Sweeney said the attack was "extremely rare for New Zealand" and "all options were open" as to whether it was planned or random.

The victim, a woman in her 20s, did not know her attacker. Her partner was at work when the attack occurred.

She phoned emergency services afterwards and was treated at the scene, then taken away for medical care

Today she will be formally interviewed and a scene examination at her address completed.

Officers today will be making inquiries in the vicinity of Middlepark Rd, trying to identify possible suspects.

Presenting in a hi-vis vest would mean the man could be mistaken for a contractor, and he appealed for anyone who saw a man fitting the description in the area between 10am and 11am yesterday to contact police.

"In Christchurch we're accustomed to seeing fluorescent jackets. People don't notice them so much," Sweeney said.

"There is an inference when someone approaches an address in a fluorescent jacket that they are a contractor."

It was too soon to say if a weapon was involved before police formally interviewed the woman, he said.

It was not known how the offender travelled to or from the victim's home.

A man who fitted the description was seen at Westfield Riccarton mall yesterday and had been spoken to, but police were confident he was not the offender, Sweeney said.

Police would maintain an active presence in the Sockburn area in the coming days.

A nearby resident, who did not want to be named, said she had just left her house for work at the time of the alleged rape.

The woman said she did not see or hear anything suspicious, but it was a busy street and she would not have thought twice if she saw someone walking by.

Another resident said he often walked his children in the area and had never seen anyone fitting the description.

St Thomas of Canterbury College was placed in lock-down for about an hour while police searched the neighbourhood.