Queensland MPs are to get a $57,000 pay rise following Crown Law advice a pay freeze four years ago was unlawful.

But the government claims it won't cost taxpayers an extra cent - as long as no member of the government past or present makes a claim for back pay.

In a rare move, acting premier Jeff Seeney released Crown Law advice to the government to explain the pay rise.

He said found former premier Anna Bligh had acted outside her powers when she froze MPs

salaries in 2009.