The public prosecutor has launched an investigation into the survivors of a deadly avalanche in the Italian Alps in which a German mother and her 11-year-old daughter died.

It must be established who played a pivotal role, the justice authorities in the city of Bolzano in the German-speaking province of South Tyrol said on Friday.

The mother and daughter from the south-western German city of Ludwigsburg were pulled alive from the avalanche on Wednesday by rescue workers, but died later of their injuries.

The father of the family - who survived - was also a part of the group of nine members of the same skiing club that had gone off piste in the Malga San Valentino ski resort area, known in German as Haideralm, in the Venosta valley.

"The public prosecutor is investigating all people who took part in the off-piste descent," the authorities said, adding that all the witnesses had been questioned.

"Because the cause of death of the two victims is clear, there will be no autopsy carried out," they said.

The extent of the avalanche was being investigated; in Italy any activity that inadvertently causes an avalanche is a crime, and if someone dies, charges of manslaughter can follow.

The mother was found under a 1-metre deep layer of snow right next to her daughter, Italy's ANSA news agency reported. The avalanche was 150 metres wide, ANSA said.