A coroner has ruled Ashburton teenager Kirsty Bentley died from a massive blow to her head on the day she went missing.

The finding comes 18 years after she died.

Aged 15, Kirsty disappeared on December 31, 1998 while walking the family dog Abby on the banks of the Ashburton River.

A short inquest was held in Christchurch District Court yesterday, Fairfax reported.

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The hearing before Coroner Peter Ryan was a legal formality, and the police investigation into Kirsty's death remains open.

Coroner Ryan ruled Kirsty died of a massive, non-accidental blow to the head, at an unknown location, on the day she went missing, Fairfax reported.

The day after Kirsty went missing, the family's dog was found tied to a tree near the river, and Kirsty's underwear was nearby. Her body was discovered two weeks later in the Rakaia River gorge, 40km away.

No one has been charged over Kirsty's death, which remains a cold case.

Her father, Sid, was a suspect in his daughter's murder but denied any involvement. He died of cancer last year aged 64.