Start from the "rule of 220 billable days": a standard working year has 260 days (52 weeks x 5 days), but you don't get paid for all of them as a contractor. Subtract 20 days of annual leave (about 7.7% of the year), 10 public holidays (about 3.8%, though this ranges 10-13 depending on your state), and 10 sick or carer's days (another 3.8%), and you land at roughly 220 billable days, before accounting for any gaps between contracts.
From there, your rate needs a loading on top of the equivalent permanent salary to cover what a permanent employee gets for free: paid leave, superannuation, insurance, equipment and job security. Multiply your target base salary by your loading factor, then divide by your billable days. A sole trader carrying their own insurance, accounting and self-funded super typically needs a 25%-35% loading; a contractor on PAYG or labour-hire agency payroll, where the agency already pays super, workers' compensation and payroll tax, generally only needs 15%-20% to cover the unpaid-leave gap.