A well-packed gym bag removes friction from your training routine. When everything you need is ready to go, there's no excuse not to train and no mid-session interruptions hunting for equipment or amenities.
Absolute non-negotiables: training shoes appropriate for your activity (flat, stiff soles for lifting; cushioned running shoes for cardio), workout clothing, a lock for your locker, water bottle (minimum 1L - hydration is non-negotiable for performance), and a towel (many Australian gyms require one). A quality shaker bottle and pre-portioned protein powder or BCAAs if you supplement during or after training.
Highly recommended: training gloves or chalk (for grip on pulling movements), a lifting belt if you train heavy compound movements, resistance bands for warm-ups and accessories, wireless earphones, and a training notebook or phone app for logging workouts. Foam roller or lacrosse ball for pre/post session mobility work is valuable if you have the space. For hygiene: deodorant, dry shampoo or shower essentials if you shower at the gym, and a spare set of clothes. A simple routine - packing your bag the night before training - ensures you're never scrambling in the morning and never have an excuse to leave something behind.