Detective Anthony Darvill said the man's demeanour suddenly changed and he started demanding "quite a violent sexual act" from the woman.
"She's made it clear to him she wants no part in that whatsoever," he said.
"He's taken that decision from her, he's quite violently sexually assaulted her."
A woman was subjected to a violent sex attack in Grey Lynn in September. Photo / Supplied via Police Ten 7
Darvill, appearing on the television crime show Police Ten 7, said the woman suffered "quite a serious injury".
"These are potentially life-threatening injuries that she's suffered and she's concerned that this might happen to someone else if he's not caught," he said.
"Her words to me are: 'he's going to kill someone if we don't catch him'.
After the attack the woman asked to be driven to a friend's house in New Windsor.
The man agreed and drove her along Great North Rd through Waterview, stopping once at the intersection of Blockhouse Bay Rd and St Jude St in Avondale.
"I believe that this was because he was concerned about the amount of blood that was in the back of his van," Darvill said.
The attacker dropped his victim off near her friend's house and took off.
Darvill said the attack was "horrific" and it was crucial the man was found.
The man who subjected an Auckland woman to a violent sex attack then drove her to a friend's house. Photo / Supplied via Police Ten 7
"The victim's given us a really good description of the van and i think that's the key to solving this," he said.
The man was driving a late model "candy apple red" van similar to a Nissan Caravan or Toyota Hi-Ace.
"The van had a mattress with a green sheet set up in the back and it's quite clear to her that this wasn't a tradesman's van - it was very tidy," said Darvill.
The attacker was described at Caucasian with short light brown hair.
Darvill appealed to anyone who knows the man to come forward.
