Section 137(4) of the Crimes (Sentencing) Act 2005 (ACT) requires mandatory substitution of the original sentence where an offender completely fails to comply with an assistance undertaking; the word 'substitute' bears a restorative purposive interpretation. An implied undertaking to give evidence, even without a written undertaking, is sufficient to engage s 137. A Crown appeal under s 137 for breach of an assistance undertaking is not an ordinary sentence appeal — increasing the sentence is not punishment but withdrawal of an unearned benefit.
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