The 1883 Thames St courthouse was closed in late 2011 after being classed as earthquake-prone.
Court services were relocated to the nearby Oamaru Opera House and then to a portable facility in Humber St in 2014.
After a short delay caused by poor weather, earthquake-strengthening work started in late February and involved steel support rods being installed throughout the building and the building's roof being replaced.
Its guttering was also re-contoured.
The focus has now shifted to the inside of the building, where the entire interior is being re-painted and general maintenance work carried out.
The Ministry of Justice has started to install new security cameras and other technology such as wireless internet and telephone communications, and equipment to enable audio and video links will be installed before the building reopens.
The layout of the courthouse has also changed.
The opera house side of the building will form the courthouse's reception area, front counter and meeting and interview rooms.
A judges' area will be at the rear of the building, and toilets, cells and a secure area for solicitors to speak to offenders will be on the Fat Sally's side of the building.
A tree between the building and the opera house will be removed to allow shared access with the opera house driveway.
Waitaki District Council property manager Renee Julius said when completed, the building would be a fitting space for solicitors, court staff and police to work in.
"They are actually going to be entering a courthouse that is fit for the work that they actually do and from the health and safety and security aspect, I think they will be thrilled to be back in the courthouse."
A public open day was also being planned, she said.
"I just think it will be nice that our ratepayers get to have a look around it."
While the cost of the work has not been disclosed, it was within the council's $900,000 budget, funded through its endowment fund.
After long negotiations, an agreement was reached between the council and the ministry in 2016 that resulted in the council taking ownership of the building and agreeing to lease it back to the ministry.