Police and volunteers in Cambodia are searching for a missing British woman last seen by friends in the early hours of Thursday at a beach party on the island of Koh Rong.

Amelia Bambridge, 21, from Worthing in Sussex, went to a party on Wednesday at Police Beach, which is popular with backpackers and located off the south-west coast of the South-East Asian country. Her handbag, containing her phone and bank cards, was found on the beach on Thursday but she has not been seen since the event.

Concern was raised on Thursday when Bambridge was not seen at the Nest Beach Club hostel on Koh Rong, where she had stayed and left her passport. She also failed to meet a friend for a scheduled ferry trip.

On Saturday morning police teams and volunteers searched the island and the surrounding ocean for signs of her. Bambridge’s parents have spoken to British embassy staff and on Saturday were travelling to Cambodia.

Amelia’s sister, Georgie Bambridge, 19, told the Guardian that she spoke to her sibling from the UK shortly before Amelia went to the party. Georgie said that after her backpacking trip Amelia, a former student of Brighton Hove and Sussex sixth form college, planned to train as an English language teacher in Vietnam, where her father works.

“She was telling me about travelling [and about how] she felt more confident and so much more happy,” Georgie said. “She was excited about going to this party. She’s officially missing now. The police started involvement in the search party this [Saturday] morning. They’re searching, and they’re searching the sea.”

Koh Rong’s regular Police Beach parties have a reputation for extreme hedonism. One online reviewer wrote that they were known for “free drug usage with no police, no restrictions”. Another review stated: “Some questionable things always seem to happen at the parties, so visitors should stay aware when they are there just in case.”

Ryan Harris, 18, from Worcester, befriended Bambridge in Phnom Penh then travelled with her and other friends to Koh Rong on 19 October. He went to a Police Beach party on the night he arrived on the island.

Ryan did not return to Police Beach for the party Bambridge attended on Wednesday, just before she went missing. He said he was in contact with a female friend of Bambridge’s who had been at the event with her.

“She [the friend] said that the last time she saw Amelia she wasn’t on anything,” he said. “She was sober, she’d had a few drinks. The last time she saw her she was fine.”

In 2013 a 55-year-old US tourist named Katherine Grgich was killed on Koh Rong in what police said was an aggravated robbery that escalated to murder. A local man was identified as a suspect but no one has been brought to trial for the death.

Hannah Clark, 21, a friend of Bambridge’s, was helping to raise awareness of her disappearance on social media from the UK. She described Bambridge as her “most reliable friend”, adding that she would be hugely unlikely to break off contact from loved ones voluntarily.

Ryan said of Bambridge: “She’s lovely, a really nice girl. She’s just a normal, outgoing person. She’ll happily chat to anyone. I cut her hair in Phnom Penh – there were three girls there who’d just moved into our hostel, and she was chatting away for the whole hour.

“The last time I saw her I said: ‘I’ll see you in Siem Reap.’”