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Funds from ticket sales will help stabilise the ruin of the castle on the cliff top above St Clair.
The Cargill's Castle Trust plans to maintain the ruin, which it says has a dramatic coastline setting and the potential to become an important tourist destination.
Formed in 1997, the trust has bought, fenced and tidied the site. After stabilisation plans are finalised, it will start raising funds for the work, which could cost about $1 million.
The heritage homes open day would feature eight Dunedin properties, including a large brick home that used to be in flats, a converted Presbyterian church and a City Rise house with "amazing'' stained glass, he said.
The ninth property was a modern eco-friendly home in St Leonards with "magnificent'' views.
The trust's last heritage homes open day two years ago attracted more than 400 people and organisers expected it to sell out again.
"It's always a fun day out,'' he said, adding the event was an ideal way for people to spend Mother's Day.
Tickets for the event, which runs from 11am-4pm, cost $40 and are available from Nichol’s Garden Centre and Metro Realty. The price includes morning or afternoon tea.