Section 16(3C) of the Sentencing Act 1991 creates a presumption that imprisonment imposed for offences committed on bail will be served cumulatively, unless the sentencing court directs otherwise.
The Full Court (McInerey and Crockett JJ, Gillard J agreeing) said in R v Gray  VicRp 27;  VR 225:
As to the circumstance that offences were committed whilst the applicant was on bail, it is, we think, permissible for a sentencing judge to take this into account at least to the extent of assessing the prospects of the applicant’s reformation. The circumstance that an applicant has committed offences while on bail is, of course, a circumstance justifying a court in making an order for revocation of that bail–see R v Hill,  VicRp 59;  VR 556. Bail is granted on the supposition that the applicant will answer to his bail and is most certainly not granted to enable the offender to commit further offences whilst on bail. Indeed, it has often been said that the commission of offences whilst on bail is an abuse of the privilege of bail. Whether or not bail is to be regarded as a privilege or as a right need not here be determined, though the weight of history is, we consider, in favour of the first view. The real point is that the commission of offences whilst on bail indicates contempt for or disregard of the system of law under which bail was granted to the offender; it suggests that the offender has small regard for the law and little intention of obeying its commands.
Furthermore, it may be said that the person who commits an offence on bail has abused or betrayed the confidence reposed in him by the tribunal which granted him bail. The fact that a crime has involved a breach of confidence or trust, e.g. by a solicitor or bank manager or member of the police force, has always been regarded as a matter relevant to the question of sentence–cp. R v Wright (No.2),  VicRp 17;  VR 174, at p. 181. Prima facie the quantum of sentence is dependent on the circumstances of the commission of the crime and its immediate consequences, and should not be increased by reference to events occurring after the offence has been committed. But just as conduct subsequent to the commission of the offence which indicates a clear intention to reform is a matter which the offender is entitled to have taken into account in his favour, so also conduct tending in the other direction, i.e. showing that the offender is unlikely to reform, or has at least not yet reformed, is a matter relevant to the sentencing discretion, if or in so far as it suggests that to extend clemency would serve no useful purpose or that leniency likely to be abused.
It follows, we consider, that the sentencing judge was entitled to take into account the fact that all the offences except those the subject of counts 1 and 2 were committed whilst the applicant was on bail or on probation. What weight the sentencing judge was to assign to that circumstance was a matter for him. But whether he was entitled to increase the sentence which he would otherwise have awarded by reference to the circumstance that the applicant was on probation or on bail is a matter which we find unnecessary to decide on this application. It must not be overlooked that breach of a recognizance for bail exposes the accused to proceedings under s5 of the Crown Proceedings Act 1958 (as amended by Acts Nos.7900 and 8124) with the possibility of imprisonment if the amount of the recognizance be not paid–cp. Re Baker,  VicRp 87;  VR 717; Re Condon,  VicRp 40;  VR 427. However, it must be borne in mind that such consequences follow not from commission of an offence whilst on bail but from the failure to answer to bail.
I’d always assumed that the mere fact that the offender was on bail at the time of offending is not an aggravating feature itself; rather it’s a matter to be considered when looking at other factors particular to the offender such as remorse, prospects for rehabilitation and so on.
The Judicial College’s Sentencing Manual points to a case of Basso & Frazzetto (1999) 108 A Crim R 392 that says otherwise.
In that case Chernov JA said (Batt and Charles JJA agreeing) at 21:
The approach by this Court, albeit often unstated in terms, has been to regard the commission of an offence whilst the offender is on bail as an aggravating factor. In R. v. Storey  1 V.R.359, for example, this Court considered, inter alia, the issues of the onus and standard of proof in sentencing. Winneke, P., Brooking and Hayne, JJ.A. and Southwell, A.J.A. recognised, at 365, that the fact that the offence was committed whilst the offender was on bail, could be described as a “circumstance of the offender”. At 366, however, their Honours rejected the utility of drawing a distinction between circumstances of the offender and those of the offence as a basis for “recognising proof of facts which bear upon sentence”. A relevant distinction, they said at 369, was between circumstances adverse to the offender (aggravating factors) and circumstances in favour of the offender (mitigating factors). It would follow that since the fact of the commission of an offence during the period of bail is against the interests of the offender for sentencing purposes, it is to be treated as an aggravating factor.