A NSW coroner has blamed a local council for the deaths of five family members in a preventable road collapse, and says there should be an inquiry into its competence.

Deputy State Coroner Paul MacMahon found Gosford City Council knew a culvert beneath the Old Pacific Highway had been rusting for years, but either lost or ignored engineering advice.

Adam Holt, his partner Roslyn Bragg, 29, their young daughters Jasmine, 3, and Madison, 2, and Ms Bragg’s nephew Travis, 9, died when the culvert collapsed and swallowed part of the highway at Somersby, north of Sydney, in June last year.

The Roads and Traffic Authority (RTA) handed responsibility for the road to the Central Coast council in 1995, and the council patched its surface in 2002 and 2004 after it began to sink.

MacMahon said the council knew the culvert had been rusting since a year after it was built in 1983.

“It must have been apparent, even to a lay person, but especially to a qualified engineer, that the loss of structural integrity … of the culvert would at some time lead to a collapse,” the coroner said.

“Not withstanding Gosford City Council knew that, for whatever reason, the pavement above Piles Creek was subsiding, no risk analysis was undertaken or regular inspection instituted to ensure that the public were not put in jeopardy.”

MacMahon said the inquest raised “serious questions” about the competence of council management and its engineers.

Departing from his written findings, MacMahon addressed the victims’ families, saying: “I can only say that it is quite clear that their deaths were both unfortunate and completely unnecessary.

“If appropriate service had been provided by Gosford City Council they would not have occurred.”

He called on Local Government Minister Barbara Perry to establish an independent inquiry into the council’s operations.

The coroner absolved both the RTA and Mr Holt, who was behind the wheel of the car when it disappeared into the hole, of any blame.

Ken and Gay Holt wept and embraced as MacMahon found their son had been driving cautiously but did not have sufficient time to stop.

The coroner also found the alcohol and cannabis in his system were not factors in the crash.

Ken Holt said the findings supported their belief that their son “died a hero trying to save his family”.

Outside court, the solicitor for the Bragg and Holt families said they were considering legal action against the council.

“We would ask that the council look at and adopt the recommendations to prevent such a tragedy ever occurring again,” Patrick Heath told reporters.

Gosford City Council again extended its sympathies to the families of those killed, but said it needed more time to examine the coroner’s findings.

“General Manager Peter Wilson said council will need time to review and consider the findings before providing additional comment,” it said in a statement.