The death of Ashburton teenager Kirsty Bentley will be investigated by a coroner, 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.

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

Coroner Peter Ryan is investigating the case and will hold an inquest in Christchurch in July, a spokesman said today.

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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.