State land officials today plan to inspect a hillside below the Diamond Head lighthouse after rocks — one the size of a van — fell yesterday morning, seriously injuring a woman.

The rocks appeared to be part of an overhang along the beach and did not tumble down the hill. It was unclear yesterday if the debris came from state, city or federal property.

Deborah Ward, state Department of Land and Natural Resources spokeswoman, said state land and engineering personnel are expected to investigate today.

"We are going to follow up and first of all determine whose land that is," Ward said. "If it's DLNR land then we'll have to investigate further to see what is the condition of the hill stability and what steps to take next."

The beach remained opened yesterday, although yellow tape was placed around the debris. The area is known for rockfalls and signs warning of the hazard are placed along Beach Road, which leads to the beach.

The incident happened about 8:30 a.m. along the shoreline below the Diamond Head lighthouse. A 44-year-old woman was picnicking with family and friends and was either lying or sitting when debris came down.

The woman was hit by rocks, but she was not buried or pinned, said Bryan Cheplic, city Department of Emergency Services spokesman.

"The amount of land that shifted was as big as our ambulance," Cheplic said.

Capt. Terry Seelig, Fire Department spokesman, said the woman remained conscious, but suffered head and back injuries.

Her companions called for an ambulance and then helped her about 200 yards back to the turnaround at the bottom of Beach Road, where they were met by emergency personnel.

Seelig said there were three other adults and two children in the group, but no one else was injured.

"There's a tidal pool and a strip of beach, but it was a little small at that time because of the tide so they chose to set up their beach site at the base of the cliff," Seelig said.

"While they were there, some of the cliff sluffed off," he said. "It just sort of dropped. It didn't fall from very high, but it just kind of dropped and it was a large amount."

Cheplic said the woman was in serious condition when she was taken to The Queen's Medical Center. Hospital officials declined to release information on her condition.

Reach Curtis Lum at culum@honoluluadvertiser.com.