Cormann warns against Labor backsliding

More than 50 Labor candidates are in open defiance of the government's tough line on border protection, frontbencher Mathias Cormann says.

Senator Cormann says in 2007 then Labor leader Kevin Rudd claimed he would turn the asylum seeker boats around, but after the election Labor went "weak at the knees" and put the people smugglers back into business.

"The truth is that Labor has more than 50 candidates in this campaign that are in open defiance against our strong border protection policies," he told ABC radio.

The government revealed on Wednesday that earlier this month it had turned back its 28th another asylum seeker boat, with the 21 on board returned to Vietnam.

Labor says it backs the government's border protection policies but the coalition has pointed to a succession of Labor candidates who have hedged their support.