In the 19th century, businessman and politician William Larnach and his family used this space as a breakfast room.
It was later used by the Barker family as their private sitting room, from about 1970 until the 1990s.
Then it was used for various other purposes, including storing furniture and some items linked to the castle gift shop.
Larnach Castle Ltd executive director Norcombe Barker said the sizeable, elegant "Breakfast Room" was ultimately too good to be kept out of sight.
"It’s a beautiful room. It’s such a shame that it’s taken us this long to get it up to standard.’"
Extensive refurbishments were about 95% complete, and it was intended to use the room for greeting and farewelling visitors on busy cruise ship days.
The room, with its ornately carved ceiling, detailed plasterwork and wooden panelling, could also be used for evening dining, he said.
Extensive renovations, including external work such as moving roof water tanks, replacing roof slates and various repairs, had been undertaken over the past 18 months.
About $130,000 — comprising a $55,000 grant from Heritage New Zealand and money from the family — had been spent on the renovations, which were continuing inside the room.
It was hoped to open the room to the public later this year, he said.
