Police are investigating a violent home invasion in Christchurch.

A man was detained, assaulted and stripped naked during a home invasion in Christchurch, police say.

Detective Wayne Boyd said three men went to a flat in Blenheim Rd, Riccarton, shortly after 5am on Monday.

The trio detained, assaulted and stripped naked a man who lived at the property, Boyd said.

They then allegedly sexually assaulted a woman before fleeing.

Boyd said the man who was attacked told police he had arranged to buy a cellphone from the three men.

"We are following strong lines of inquiry in relation to the people involved."

Anyone with information was asked to call police on 03 363 7400 or Crimestoppers anonymously on 0800 555 111.