The woman was walking on Collingwood Beach in Vincentia with four others, including her 71-year-old daughter, on Sunday morning.

Emergency services were called to the scene about 7am after reports three roaming dogs had attacked the group on the popular Jervis Bay beach.

The 91-year-old died at the scene.

Paramedics treated the five people at the scene - suffering lacerations and bite marks - and took three women to Shoalhaven District Hospital in a serious but stable conditions.

The three dogs, believed to be Cross Bull Mastiffs, have been seized by Shoalhaven City Council rangers.

South Coast Police District Commander Detective Superintendent Greg Moore said an investigation had begun in to the incident and circumstances surrounding it.

He praised the efforts of bystanders and emergency services who restrained the dogs while the victims were being treated.

Detective Superintendent Moore said it was reminder to all dog owners to keep their pets secured.

"In broad terms, there are criminal offences associated with keeping dangerous animals or not securing dangerous animals" he said.

"That is one aspect of the investigation will be looking at; how these animals got out of their premises."