The report, showing two-yearly intervals from 2004 to 2012, saw a considerable decline over the first two years.

There was then a spike in the retention rate - which occurred in all but the the Northern Territory - in 2010.

However, Tasmania declined again in 2012.

Both Tasmania and the NT showed a clear decline over the eight-year period. Victoria's retention rate declined slightly.

The remaining states improved as did the national retention rate.

State Education Minister Brian Wightman said Tasmania must continue to improve its retention rate.

He said that although the current retention rate was below the national average, there had been a significant improvement since 2008.

"Improvement over this period for Tasmanian government schools is also greater than the rate of improvement nationally in government schools," he said.

He said the data was a measure of the number of full-time students in year 12, expressed as a percentage of those in year 10 two years earlier.

He said the state also had a high percentage of part-time students.

Mr Wightman said the Liberals would destroy colleges if all high schools were extended to year 12 without a clear plan or proper costings.

Opposition education spokesman Michael Ferguson said the report demonstrated why the state's public schools had to be extended to year 12.

"Apparent retention rates for year 10 to 12 students - full time - are again the lowest of any state," Mr Ferguson said.

"What is even worse is our retention rate started going backwards when the Labor-Green minority experiment began in 2010."

He said the 10-year plan to extend high school to year 12 was a central plank for improving retention and making the state job-ready.

Mr Ferguson said the Liberals had budgeted $45 million to extend 21 regional high schools in the first four years.