The society has a roll of up to 40 members and operates a small history museum in the town.
Mrs Underhill is preparing a submission to the council, calling for the retention and restoration of the building in Holloway St.
"There has been council money spent on the library and the railway station, and Rotary spent money on our band rotunda. All it will take for the Old Courthouse is some money and the will," she said.
"It is part of the history of Carterton and it's a treasure that we need to preserve.
"We have so few treasures here and we really need to protect what we do have.
"It must not be left to become a white elephant either, but should be restored and made somewhat self-sufficient, with toilets and modern kitchen facilities and wheelchair access, and run under the auspices of the events centre. It will be too late once it's thrown down the dump."
She said a restored courthouse could benefit numerous organisations in the town - for a nominal fee - and possible uses could include activities as diverse as council committee meetings and small wedding ceremonies.
"It is located in a central and well-lit area and could be turned into very pleasant space for community use," she said.
"We don't have to hang on to everything, I know, but some things - like our Old Courthouse - are treasures that simply must be held on to and protected."
The facade would enhance "the township heritage" along with the Vogel railway station (1880), the library (1881), St Mark's Church (1875) and Wakelins' Flour Mill, which is being revamped.
Tenders were called for the construction of the Carterton Courthouse and Police Stables in 1883 and the building was designed by assistant colonial architect Pierre Burrows, whose most important buildings include the Post Office at Christchurch and Mount Eden Prison.
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 courthouse, if upgraded, would warrant and reclaim a Historic Places Trust listing, Mrs Underhill said, which was lost because of alterations and shifts.
The building was used as a courthouse until 1951, when it was disestablished along with a number of 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 Holloway St facade has been kept predominantly in its 1902 form, which differs slightly from its original.
The building is not heritage listed, but is listed on the Carterton District Council schedule of heritage buildings. The Historic Places Trust had shown some interest in the building, due to its original architect and function.