The Australian government says it has turned back 28 boats to prevent asylum seekers from reaching Australia during three years in power, with the latest carrying 21 Vietnamese intercepted this month.

The conservative coalition government on Wednesday warned that the boats would start coming from Indonesian ports in greater numbers if the center-left Labor Party wins a national election set for July 2.

Border Protection Minister Peter Dutton said the 21 Vietnamese, including four children, where returned to Vietnam after their refugee claims were assessed and rejected at sea. He declined to give details of the interception.

Opposition leader Bill Shorten says a Labor government would maintain the same tough policies that have prevented any asylum seekers from reaching Australia by boat for two years.