Police have located and spoken to a key witness to a double-fatal crash in South Canterbury on Boxing Day.

The woman was located through the assistance of the public, after police appealed for help yesterday.

The two-car crash happened around 8.35am on the Waimate Highway at Glenavy in South Canterbury on Tuesday December 26, 2017.

Horton James Hill, 80, of Christchurch and 31-year-old Matthew James Gilchrist of Oamaru died at the scene of the crash between Carrolls and Stangers Rds.

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Police said they had received information from another witness that a female driver of a red car was also at the crash scene, but left before emergency services arrived.

Mid-South Canterbury area commander Inspector Dave Gaskin of Timaru said it was "pretty apparent" the northbound vehicle was travelling at well over the 100km/h speed limit when the crash happened.

"If you want to be hard-nosed and callous about it - if he was travelling at the speed limit, he would still be alive, and so would the other gentleman," Gaskin said.

"It was obviously an incredibly high-impact crash. Two vehicles coming together at 100km/h in opposite directions produces an awful lot of kinetic energy - but one of the vehicles was travelling well in excess of that speed.

The driver of the northbound vehicle that split in half was thrown from his car.

The southbound car was taken to Timaru so firefighters could free the driver's body.