The repairs include replacing the front door, flooring and joinery and making the exterior weather-tight.
It was expected work on the porch would be completed this week, with the painting taking a further two weeks.
The courthouse will be painted in "heritage" colours, which would tie in with the library's colour scheme, Mr Gillard said.
Once fully renovated, the courthouse will act as a community hub, with community agencies making use of the space.
Mayor John Booth said the courthouse would complement the adjacent events centre.
"It will add additional space for meetings or whatever they want to use it for."
"They are a dedicated group of people and it will look marvellous once it's done."
The initial investigation into the renovations was funded by the council, but since then the trust has been relying on private donations, which have paid for the alterations to the porch.
The trust is soon to be formally registered as a charitable trust and once this step has happened organisers hope to apply for community grants to help fund the renovations.
The cost of the renovation has yet to be finalised.
The Carterton Courthouse was built in 1884 and was originally located on the Post Office site, on the corner of Holloway St and High St.
In 1902 it was relocated to the site now occupied by the Council Chambers building and modified slightly.
The building was used as a courthouse until the 1950s, when it was disestablished with several other small courthouses in the region, and moved to its present council-owned site.
Ownership of the building was passed to the Order of St John, which in 2011 passed the building to Carterton District Council.
The building is not heritage listed, but is listed on the Carterton District Council schedule of heritage buildings.
-Those interested in donating to the renovations can post cheques to PO Box 21, Carterton 5743 or get in touch through the trust's Facebook page: Carterton Courthouse - Community Hub or: cartertoncourthousetrust@gmail.com.