A Darling Point deceased estate with sweeping harbour views and a $10 million price guide is heading to auction — and it comes with a notable back story.
The whole-floor apartment at 3/8A Carthona Avenue belonged to the late Judith Mactaggart, mother of low-profile Brisbane investor John Mactaggart, whose family shareholding in ASX-listed enterprise software company TechnologyOne has placed him squarely in billionaire territory. The property is set to go under the hammer on 1 September, handled by Ric Serrao and Richard Faludi of Raine and Horne Double Bay.
Judith and her late husband Dugald purchased the apartment in 2000 for $3.4 million. At today’s guide of $10 million, the growth in value over a quarter-century speaks to just how far inner-harbour Sydney property has come — and how tightly held addresses like Carthona Avenue remain.
The apartment itself is an impressive proposition. Occupying the entire third floor of a boutique four-apartment block, it spans more than 420 square metres on title and offers north-easterly views stretching across the harbour towards Manly and The Heads. A full-width terrace makes the most of that outlook, while inside, the generous floor plan includes three bedrooms, two bathrooms, a large open-plan living and dining area, a granite kitchen with Ilve gas appliances, and a master bedroom with a spa ensuite and private balcony. There is also a dedicated gym, home office, high ceilings, ducted air conditioning, designer bathrooms, and a grand entry foyer with polished parquetry timber floors. Garaging for three cars is included — a rarity in this part of the world.
Carthona Avenue is widely regarded as one of Darling Point’s most exclusive addresses, and the building itself — one of only four whole-floor apartments, designed by Giles Tribe Architects and built in 1996 — has long attracted buyers seeking privacy, space, and harbour proximity in equal measure.
As for John Mactaggart, 66, he has maintained a deliberately low profile despite his considerable wealth. He has been a director of TechnologyOne since 1987 and was a founding shareholder, with his family’s early investment considered integral to the company reaching the scale it has today. The Mactaggarts — John and his late father Dugald — backed TechnologyOne co-founder Adrian Di Marco in 1987 when the business was still in its earliest stages. According to reporting by The Australian in June 2025, the family’s stake had grown to be worth around $930 million at that point, though sharemarket movements since then have seen that figure fluctuate. Mactaggart himself has said he has little interest in wealth rankings, and has been candid about his reluctance to sell down his shareholding given the capital gains tax implications of doing so.
Whether the Carthona Avenue apartment attracts a buyer equally comfortable with the long game remains to be seen — but with a price guide of $10 million and a 1 September auction date, the harbour-view penthouse-style home is unlikely to linger.
Published 6-August-2026
