This is a bit old, but I thought I write about it anyway:
It seems that the Australian Fair Works Act 2009 does not provide for employees wishing to return early from parental leave. Sure, the employer can consent to an employee returning early. But there is no legal right for the employee to do so.
Interesting enough, the NZ Parental Leave and Employment Protection Act considers such a scenario. Section 45 of the Act provides that in certain circumstances (for instance if there was a miscarriage or adoption), an employee can give the employer 21 days notice to return to work. Note though that this only applies if the position had been kept open. If the employer has filled the position for the period of absence, the employee’s request to return to work only triggers the respecective period of preference. Which means that if during the subsequent 26 weeks a suitable position becomes available that is substantially similar to the position the employee held before commencing parental leave, the employer would need to offer this position to the employee during that period.