Mrs Barker spent several hours gardening at the castle yesterday.
She also received a letter from the New Zealand Gardens Trust, announcing the new international ranking and congratulating her on "a wonderful achievement".
The castle garden, the Dunedin Botanic Garden, and the Glenfalloch Woodland Garden had earlier been declared gardens of national significance by the trust, which was established by the Royal New Zealand Institute of Horticulture.
The trust this year instituted a new "top tier" category, designed to recognise "New Zealand's most outstanding gardens".
The castle garden is the only one in the South Island to achieve that international ranking, with three privately-owned North Island gardens, "Richmond" in Carterton, "Te Kainga Marire", in New Plymouth, and Ayrlies Gardens, in Auckland, also honoured.
Mrs Barker was "thrilled" with the outcome, which vindicated the efforts of many people over the years, including head gardener Fiona Eadie and groundsman Gary Drake: "It's been a journey, and a lot of people have been part of that journey."
When she first saw the castle about 40 years ago, its 14ha grounds were an overgrown wilderness, since restored and revitalised.
An overgrown rock garden, created in the 1930s, was discovered, and lovingly brought back to life.
A South Seas Garden has been developed over the past seven years.
Visitors did not need to be gardening experts to enjoy themselves: "It's about the atmosphere, and the history, and being with nature."
