A 24-year-old man with known asthma presents with wheeze and shortness of breath. He has used his salbutamol inhaler 10 puffs in the last 2 hours with little relief. On examination: RR 28 breaths/min, HR 110 bpm, speaking in full sentences, SpO₂ 95% on room air, widespread wheeze. According to the Australian Asthma Handbook, what is the most appropriate initial treatment in the emergency department?