when is it acceptable to tailgate another vehicle

Short answer: never. In Australia you must keep a safe following distance so you can stop without punting someone’s bumper; think the 3-second rule in the dry and 4+ seconds in rain, at night, or when towing. Tailgating is illegal and attracts fines and demerit points in every state, plus a generous helping of insurance pain if it goes pear-shaped. If someone’s glued to you, keep calm, maintain your speed, and merge left when safe—don’t brake-check unless you enjoy panel beaters and paperwork. Heavy vehicles need even more space, and at 100 km/h that gap should be a proper cricket pitch, not a courtesy nod. Consider adaptive cruise as your adult supervision. Consult official documentation for exact figures.