MELBOURNE, Australia 

Australian Prime Minister Malcolm Turnbull and his immigration minister confirmed Wednesday that a vessel carrying 21 Vietnamese nationals was the latest of 28 asylum seeker boats turned back under their ruling Coalition.

News broadcaster ABC reported that Peter Dutton told reporters that the "unseaworthy" boat was intercepted at the beginning of this month and its passengers sent back to Vietnam by plane.

"They claimed that they were wanting protection, it was found that they were not owed protection and they were returned to Vietnam," he said.

Under its hardline immigration policy, Australia detains asylum seekers who arrive by boat, in processing centers on the small Pacific islands of Nauru and Manus, where conditions have been described as appalling by rights advocates.

Turnbull told a separate press conference Wednesday that Australian authorities were aware that another asylum seeker boat had departed from Indonesia’s northern Aceh province, describing such developments as proof that the “challenge of people smuggling is greater than it has ever been".

"The people smugglers test us… they know where we stand,” he said.

“Imagine what they will do to a Labor government," he warned as Australia prepares for a July 2 election, adding that the opposition was "divided" over border protection.

"People smugglers in Indonesia are watching very closely and they believe if there's a change of government on July 2, the people smugglers will be back in business and people will be back on boats coming to our country," he said.

Opposition Leader Bill Shorten refuted the claims of Turnbull and Dutton, saying they “should be ashamed of themselves, sending out a signal to people smugglers that there is a lack of national will to deter people smugglers”.

In July last year, the Australian Labor Party formally reversed its position on turning back boats, eliminating one defining difference between their asylum seeker stance and that of the Coalition, which has been ruling for three years.