Hospital nursing rosters typically run on rotating shift patterns, and the penalty rates attached to different shifts are one of the more significant, and least understood, parts of total take-home pay in acute care work.
Most acute hospitals run some version of early (AM), late (PM), and night shifts, often on an eight-hour or, increasingly, a ten or twelve-hour rotating pattern. Twelve-hour shifts have become more common in some units because they mean fewer handovers and more full days off per fortnight, though they suit some nurses far better than others physically.
Because penalty rates depend on the specific mix of shifts you actually worked, two fortnights with the same total hours can produce noticeably different pay if one included more night and weekend shifts. It's worth checking your roster against your payslip periodically, particularly early in a new role, to make sure penalty rates are being applied correctly.
Most units have some flexibility to accommodate shift preferences, particularly for staff with caring responsibilities, though it varies by unit and staffing levels. It's a reasonable thing to raise directly with your nurse unit manager rather than assuming the standard rotation is fixed and non-negotiable.