A NINE-YEAR-OLD girl called police after the shooting murders of her mother and her mother's partner in Tasmania.

The bodies of a woman, 31, and a man, 34, were found at a property on Hamilton Plains Road at Hamilton, a small rural town 70km from Hobart, about 5am (AEDT) today.

Detective-Inspector Colin Riley said the alleged gunman, a 59-year-old man, was previously in a relationship with the dead woman.

He was found at Lachlan, 45km away, lying beside a car pulled over to the side of the road, with a gunshot wound to the head.

The man's one-year-old son, whose mother was the dead woman, was unharmed and found inside the car.

Det Insp Riley said the man's gunshot wound was inflicted shortly before or just as a police car arrived.

The man is under police guard at Royal Hobart Hospital in a critical condition after surgery.

Det Insp Riley said the nine-year-old girl called police using a mobile phone.

''It appears to me that the nine-year-old heard what occurred in the house and we are still ascertaining ... what was seen by that child,'' he told reporters at Hamilton.

Det Insp Riley said the dead couple had been in a relationship and living in the Hamilton house together for about four months.

He said there had been no reports of violence involving the dead woman and her alleged killer.

The children, including a third child who was not in the house at the time of the incident, were being cared for by their grandparents.

Det Insp Riley said the shooting had shocked the close-knit community.