The effective constraint on the combined discount for a guilty plea and assistance to authorities under s 23 of the Crimes (Sentencing Procedure) Act 1999 (NSW) is the statutory proportionality requirement in s 23(3) — that the lesser penalty must not be unreasonably disproportionate to the nature and circumstances of the offence — not a rigid mathematical cap of 40% or any other fixed percentage. A sentencing judge who proceeds on the basis that a combined discount cannot exceed 40% absent exceptional circumstances commits an error of principle. Separately, where unchallenged expert psychiatric evidence diagnoses a mental illness such as schizophrenia, the sentencing judge must consider its potential causal role in the offending and its relevance to moral culpability, general and specific deterrence, and the weight of custodial sentence, in accordance with the De La Rosa principles.
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