A 70-year-old man presents with several days of vomiting and diarrhoea. He is hypotensive at 90/60 mmHg, tachycardic 110 beats/min and has dry mucous membranes. Serum creatinine has risen from 90 to 180 µmol/L over 3 days. There is no haematuria or proteinuria. According to Australian AKI guidance, what is the most likely classification and initial management?