More than 140 people took part in the search for Amelia Bambridge, the 21-year-old British backpacker last seen at a beach party on the Cambodian island of Koh Rong, as her parents arrived in the south-east Asian country on Sunday.

Local volunteers joined expats, tourists and Cambodian officials including police officers, divers and soldiers in searching for Bambridge. Three government boats aided the sea search, while people scoured terrain on foot and motorbikes.

Linda Schultes, Bambridge’s mother, arrived in Cambodia from the UK and is expected to land on Koh Rong on Monday morning. Her father, Phil Bambridge, arrived on the island after travelling from Vietnam, where he lives, on Sunday night.

On Wednesday, Bambridge, who travelled to Cambodia alone but forged friendships with other backpackers, left the Nest Beach Club hostel on Koh Rong, where she had been staying, and attended a party on the small, privately run Police Beach. She was last seen by friends at about 3am on Thursday at the party.

Concern was raised when Bambridge failed to check out of the hostel and meet a friend to catch a ferry. Her backpack, containing items including her bank cards, cash, phone and headphones, was found on rocks by Police Beach later on Thursday morning.

Ben Warner, 33, founder of Nest Beach Club and leader of the volunteer search effort, told the Guardian on Sunday night: “The bag that was found was very close to the shoreline, so that made us very worried. It’s unclear why the bag would be there. You’d think if she went swimming, she would have left clothes by it. That, for us, meant that there was a real risk that she could be in the water.”

Warner said that beyond the sea, jungle areas were perhaps the most challenging around Koh Rong to search. “It’s dense jungle, it’s confusing, it’s easy to get lost, there are poisonous snakes and it’s easy to get dehydrated,” he said. “It’s not the place where you just want to send people off looking.

“We’ve had to be really careful about volunteers, making sure they’re safe. It’s not just a case of saying to tourists: ‘There’s somebody lost, can you go over there and look.’ They have to be in groups, at a minimum in pairs. We have to have someone who knows the land with them.”

Police have taken witness statements, including from friends of Bambridge who were with her at the Police Beach party. Some critics have claimed online that the events feature illegal drug use.

Igor Bidani, 26, who works at the parties, said such claims were unfounded. “Normally I’m on the gate, and I control the people that come in,” he said. “If there are people making problems I send them away.”

He said Wednesday’s party was, “A really chilled night... [with] maybe less than 100 people.”

Bambridge, from Worthing in West Sussex, is believed to have visited Police Beach that night in a group of about eight friends. The walk from Nest Beach Club to Police Beach takes 30-40 minutes, and requires travelling through unlit areas of jungle. By 6.30pm on Sunday some sections of the route were in near-complete dark.

Bidani said police had temporarily banned some of Koh Rong’s most popular party nights, including Police Beach’s, in light of Bambridge’s disappearance.

He added that even without the official ban his team would have cancelled the parties out of respect, as the search continued. “The energy, the vibe… we cannot make a party after a story like that,” he said. “Not until we’ve found the girl.”