Six Chinese men who tried to enter Australia's northern waters by boat have triggered border and biosecurity operations in Australia and Indonesia.

Key points: The six men, from Jiangsu province in eastern China, flew into Bali on New Year's Day

The six men, from Jiangsu province in eastern China, flew into Bali on New Year's Day In the city of Kupang, they paid $1,000 each for an Indonesian boat and crew to take them to Australia

In the city of Kupang, they paid $1,000 each for an Indonesian boat and crew to take them to Australia They drew the attention of Operation Sovereign Borders officials when they reached Ashmore Reef, just inside Australian territorial waters

The ocean crossing through the Timor Sea in unseaworthy fishing boats is perilous at any time, but coming amid the global coronavirus scare only added to the extreme difficulty of completing the journey.

The six men and their two Indonesian crew are being detained by police off a naval base in Rote Island in the East Nusa Tenggara province, effectively quarantined on their wooden boat as a precaution against the virus.

Rote marks the end point of an ill-fated expedition for the Chinese, who all arrived in Bali from their home province of Jiangsu in eastern China on New Year's Day.

According to Indonesian police, Fan Shenghong, Cui Hennggo, Hang Yongsheng, Wang Sisen, Han Baolin and Chu Kaishan soon left Bali for East Timor before returning to Indonesia days later.

In the East Nusa Tenggara provincial capital of Kupang, the men succeeded in securing a boat operated by two local crew for what Rote police deputy commissioner Bambang Hari Wibowo said was a fee of about $1,000 each.

Later in January, the Jiangsu six set sail for Australia on their modest boat, only to draw the attention of Operation Sovereign Borders officials when they reached the remote Ashmore Reef atoll just inside Australian territorial waters.

"In the Australian waters, they were intercepted by a Border Protection vessel and [were] returned back as soon as they found Chinese onboard", Mr Wibowo told the ABC.

"They [Border Force] worry about the virus threat."

The men secured an Indonesian boat and two crew to take them to Australia's northern waters. ( Supplied )

Border Force has been approached for its description of the intercept, but being part of the secrecy regime surrounding Operation Sovereign Borders, a public account of its techniques is unlikely.

What is known is that the Chinese men were next seen back in Indonesia's 12-nautical-mile territorial waters, their boat drifting on a now empty tank of fuel.

This fits with previous methods under Australia's official border security policy of "boat turn-backs", where sufficient fuel is made available for intercepted boats to safely return to Indonesian waters, but not enough to reach further towards Christmas Island or the Australian mainland.

Off Rote Island, Indonesian police and the navy approached the Jiangsu six cautiously as potential sources of a biosecurity threat, if any were carrying the coronavirus.

Several officers were seen wearing thin surgical masks to shield themselves from any contagion.

"As we identified they were Chinese, we then contacted the health office to make sure they were not having the corona infection syndrome", Mr Wibowo said.

"So far they are clear of the symptoms."

Arrangements are being made to transfer the young men from their boat and to hand them over to immigration officials for questioning in Kupang.

The immigration department's investigation will examine the men's reasons and motives for undertaking the trip and whether a people-smuggling syndicate helped organise it.

Although Rote police believe this is the first such intercept of a would-be venture in their waters for three years, attempts by other Chinese people have been made more recently in other nearby ports.

In mid-January 2018, seven Chinese nationals were found in a boat off Kupang.

The Australian Government maintains that Operation Sovereign Borders continues to prevent "successful" arrivals of "illegal maritime ventures".