A 26-year-old woman presents to the emergency department with acute dyspnoea and wheeze. She has a history of asthma and has used her salbutamol inhaler 20 times in the last 24 hours. On arrival she is speaking in short phrases, respiratory rate 32 breaths/min, pulse 118 beats/min, SpO₂ 93% on room air and PEFR is 45% of predicted. According to Australian acute asthma management guidelines, which is the most appropriate immediate treatment?