Dorset County Hospital's (DCH) minor injuries unit is being relocated to Weymouth on a temporary basis.
The hospital has been given £62 million in government funding to expand its emergency and intensive care facilities to meet demand.
Work on the accident and emergency department began last month, though a spokesperson for the DCH said they did not know how long it would take.
This means the hospital's minor injuries unit (MIU) has been moved to Weymouth Hospital.
Weymouth's MIU is extending its opening hours until midnight, to deal with the added number of people.
The DCH's emergency department also remains open to those with serious and life-threatening conditions throughout the work.
Those with less serious conditions will be booked into the Weymouth service.
As part of the work, a new multi-storey car park with more than 650 spaces for cars will be built for the DCH, off Williams Avenue. It is hoped the project will be completed by summer 2022 and help ease pressures on the hospital caused by rising numbers of patients.
The emergency department was built to treat around 22,000 patients a year, but was dealing with almost 50,000 patients before Covid-19 pandemic began.
The new cark park will also free up land in order to expand the emergency department and Intensive Care Unit and create an Integrated Care Hub.
The relocation comes as Dorset's network of minor injury units reopen today for pre-booked appointments.
These include The Victoria Hospital, Wimborne; Swanage Hospital; Blandford Hospital; Portland Community Hospital; Westminster Memorial Hospital, Shaftesbury, and The Yeatman Hospital, Sherborne.
People are being asked to call for a telephone assessment and not to just turn up - as has been the case throughout the pandemic.
Jane Elson, Dorset HealthCare’s service director for integrated community services, said: “We’re delighted not only to re-open all our MIU services, but also to provide them in a more targeted, efficient way.
"We had to change things over the past year, and pre-booked appointments worked extremely well.
"It will save people travelling when they don’t need to, and ensure patients who do require face-to-face appointments won’t have to sit around waiting for very long.”
NHS officials say people should call 111 or visit 111.nhs.com online, and that if you require a visit to the MIU, you can be booked in.