The first case involves three prisoners sentenced before the passage of a 2011 law that toughened parole conditions.
The prisoners argued they should have been treated under the old law, which allowed early parole after serving one-sixth of a sentence.
They won in lower courts, but the Supreme Court agreed to hear the attorney general's appeal.
The second case deals with credit for pre-trial detention.
Level Aaron Carvery was arrested on the street after midnight for breaking curfew. Police found about five grams of crack cocaine in his hooded sweatshirt and $110 in cash in his pocket.
Carvery spent more than nine months in custody before eventually being sentenced in June 2011. The trial judge credited him with 1.5 days for each day in detention before sentencing, which the federal government is appealing.
The Conservatives have sought to crack down on early parole for non-violent offenders and tougher pre-trial custody and sentencing provisions for repeat and violent youth offenders.
As is its usual practice, the high court did not release reasons for its decision.
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