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A possible blood spot found at a public toilet near where a missing Christchurch woman's crashed car was found is to be forensically examined today.

Police have been searching for Emma Campbell, 28, since Saturday morning when her car was found over a bank near Dyers Pass Road in the city's Port Hills.

Her wallet was in the car, and her shoes were on a nearby rough path, but ground and air searches of the area have failed to find her.

A spot of what appeared to be blood in a public toilet was pointed out to police yesterday while they were getting security camera footage from the nearby Sign of the Kiwi cafe, The Press reported.

"A woman mentioned there was some substance in the toilet we may be interested in," Detective Senior Sergeant John Rae said.

"It was reported to be blood, but it's only one drop, so we don't know yet."

Her said police would also look at the video footage from the cafe today.

Footage recovered from a Riccarton Rd Shell petrol station showed Ms Campbell putting petrol in her car about 5.30am on Saturday. Her car was found down the bank with minor damage about two hours later.

Mr Rae said police wanted to speak with anyone who might have seen Ms Campbell's car between 5.30am and 7.30am on Saturday.

"We have got so many questions and no way to form a definite opinion of what's happened," he said.

"We still don't know with absolute certainty that she was in the car [when it crashed].

"We have not ruled out [something sinister], but there is nothing to indicate that's a hot option."