Prime Minister John Key says a new sentencing toolbox for judges dealing with the worst young offenders will be at the heart of youth justice legislation being introduced to Parliament on Wednesday.

"We need to deal more effectively with the growing group of young Kiwis who are seriously and repeatedly breaking the law. These ticking time bombs need to be sent a message that their behaviour will not be tolerated."

Mr Key signalled the Government's new approach in a speech he delivered in January last year.

The changes are aimed at the worst 1,000 young offenders. They give the Youth Court a range of new powers, including more effective sentencing options.

"We must work to break the cycle of offending earlier. The Government plans to pass this into law by the end of this year and to begin implementation as early as April. The implementation will be complete by the end of 2010."

The legislation gives the Youth Court the power to issue a new range of compulsory orders and it extends the court's jurisdiction to include 12 or 13 year olds accused of serious offences.

Repeat offenders and those who breach community orders may be subject to judicial monitoring, with the court able to ‘spotlight' certain elements of their sentence. A small number may be monitored electronically.

"Initially up to 40 of the most serious young offenders will be required to take part in military-style activity programmes run by the army, consisting of up to three months residential training, using army-type facilities or training methods.

"Other programmes will also draw on the military-style training. For example, Supervision with Activity will have up to 175 places available, and many of those young people will benefit from the lessons provided by military style training and discipline."

The changes associated with the reforms will cost up to $35 million.

"It's an investment worth making, as there are many long-term economic and social benefits to be gained by rehabilitating young offenders. This is about encouraging positive habits and behaviour."

Mr Key says the Youth Justice legislation is one of a suite of new law and order policies that are being implemented, or introduced in the first 100 days of the new Government.