A sailor on board the first Chinese yacht to enter the gruelling Sydney to Hobart yacht race is now presumed dead, after being swept overboard while en route to Australia.

Sai Jun, 23, was lost at sea late on Sunday off the coast of Vietnam while the crew were battling strong winds, his sailing club said in a statement on its Facebook page.

He was still missing on Wednesday, three days later.

"The search operation to locate Ark323 crew member Sai Jun continued today October 28," the Noahs Sailing Club statement said.

"Three ships are searching the area, and merchant and fishing boats are alerted to keep sharp lookout ... Singapore MRCC in involved in coordination."

In a milestone for Chinese sailing, the club's 52-foot Ark323 entered the race to cross the Bass Strait — which is notorious for its demanding conditions — this coming December.

The missing bowman and navigator is from the Chinese region of Ningxia, thousands of kilometres inland, but discovered sailing at university, the Shanghai-based club said.

"There's danger in everyday life, even when we cross the road," it cited him as saying after a South China Sea typhoon killed one sailor and left another four missing earlier this month.

"I want to be a sailor because I want to see the world. We need to take some risks sometimes."

The Ark323 had left the Vietnamese port of Nha Trang on the day of the accident, bound for Bali en route to Australia, the club said.

Searchers had not been able to find him, nor had a private helicopter hired by the club.

Sai was wearing a lifejacket with a personal locator beacon, but no active signal could be traced.

The rest of the crew arrived this afternoon at Nha Trang port after three intense days searching for their colleague.

"I believe he's still out there. We're all still waiting for him," his older brother Sai Bin said.

His parents will travel to Vietnam to seek further information once their documentation is arranged, he added.

ABC/AFP