Guests at a fundraising luncheon at the landmark were shown plans by leading heritage architect Ian Stapleton and heritage expert Maisy Stapleton.

The works, for which a development application has just been lodged with Orange City Council, are expected to cost up to $6 million.

Mr Stapleton, whose work in Sydney has included the re-design of the Sydney GPO and the Woolloomooloo finger wharf, said Duntryleague was one of the most significant buildings in regional NSW.

"It's really the top drawer of the 19^th century mansions built in NSW," he said.

Mr Stapleton said the lift would assist guests and staff.

Other works include refurbishing the bedrooms, adding three new guest rooms and altering some rooms to make them more usable for functions and events.

However, he said the heritage character would not be reduced by bringing the building up to date.

"We can get the lift in without spoiling anything original," he said.

"The lift makes it more attractive to people to stay. While people like to walk on the stairs, when you arrive and leave you need a lift for the luggage.

"The idea is to mix new with old in an interesting way."

Ms Stapleton, a former deputy director of the National Trust and the curator of Elizabeth Bay House in Sydney for 10 years, said a selection of heritage colours, textiles and patterns for decorating the roooms had been prepared.

She said while few photos existed of Duntryleague to show what the interior was like in its early years after being constructed for the Dalton family in 1876, the designs were suited to that period.

"[It's] a totally strong colour scheme without being totally wild," she said.

Duntryleague Mansion Foundation committee member John Kierath said they needed to raise up to $6 million for the project to proceed.

He said they would seek federal and state government grants and were hoping donations totalling $1 million could come from the Orange community.

Mr Kierath said they hoped the upgrade would increase the occupancy rate of the guesthouse from 65 per cent to 85 per cent.