The busiest emergency department in Victoria is not in the city. It is on Cooper Street in Epping, where the Northern Hospital treats more than 110,000 emergency patients a year by Northern Health's own count, in a catchment that has been adding residents faster than it has been adding hospital.

On Monday the first stage of the fix opened its doors. A four-storey Ambulatory Care Centre began seeing patients from 3 August, offering allied health, pathology, respiratory medicine and dialysis — specialist care for people who need a hospital but not a bed overnight.

It is worth being clear about what has and has not opened, because the headline number attached to this project belongs mostly to work that has not happened yet. The building that opened is outpatient care. The new emergency department is stage two, and it is not due until late 2029.

The whole redevelopment is worth $813 million, delivered under the Victorian Emergency Department Expansion Program. Northern Health describes the centre now open as housing administration, clinical and outpatient services "decanted from the old hospital" — which is the unglamorous but necessary first move, clearing space in the existing building for what comes next.

What comes next is the substantial part: a new emergency department and an inpatient tower fronting Cooper Street. The government says the finished project will bring 200 treatment spaces, including additional inpatient beds, a dedicated paediatric zone, a specialised mental health and alcohol and other drugs hub, and an emergency observation unit. It expects the completed hospital to handle an extra 30,000 emergency patients each year.

"This major investment will support the growing northern suburbs for decades to come," said Melissa Horne, the Minister for Health Infrastructure.

The scale of the existing load explains the urgency. Beyond the emergency department, Northern Health cares for more than 114,000 admitted patients a year across a network that also takes in Broadmeadows Hospital, the Bundoora Centre, Kilmore District Hospital and community hospitals at Craigieburn and Mernda. The Epping campus is the acute centre of that system.

For residents of Melbourne's northern growth corridor the practical change is modest for now and significant later. From this week, an outpatient appointment that once meant a trip into a crowded main building happens in a purpose-built centre next door. The part that matters most on the worst night of your year — the emergency department itself — is still three years away.