When you say the Northern part of Sydney? I can't tell you much about Manly or Middle Harbour. Bit further North is Narrabean Lakes then further North you'd be looking at Governor Philip Park or Sandy Point, both at Palm Beach. All would be flat enough to uphaul if you pick the conditions but all suffer from lack of parking.
Up Gosford way there is a good group sailing out of Gosford Sailing Club and Saratoga SC. Probably a bit better access to Brisbane Waters too with more options for launching.
From there the only other area I've sailed is Lake Macquarie. Heaps of choices there.
That's the extent of my knowledge