The island the maps call North Stradbroke is, to its traditional owners, Minjerribah. The Quandamooka people, including the Nunukul, have belonged to Southern Moreton Bay for an immense span of time, with archaeological evidence of occupation reaching back more than twenty thousand years. When the colonial machinery arrived in the 1820s it treated this country as a convenient transit point, a place to break bulk and store goods, and the people already here as an obstacle. Colonial records note Aboriginal resistance in the same flat, dismissive tone they reserved for bad weather and smuggling. But the deeper truth of this shoreline is that the causeway is a recent thing laid across an ancient home, and the longest story here is not the convicts' but the Quandamooka's.

The whole reason for Dunwich was a sandbar. The Moreton Bay penal settlement, established in 1824 and soon relocated up the Brisbane River, had a problem: large ships could not cross the bar at the river mouth. So in 1827, on Governor Ralph Darling's orders, Captain Patrick Logan built a military post and stores depot at a spot then called Green Point. Big vessels would unload here at Dunwich; small cutters would ferry the cargo over the bar and up to the settlement. The stone came from nearby Doctor's Hill, hauled and fitted by hand using building techniques the convicts had carried from Europe. Many of those convicts were tradesmen, quarrymen and stonecutters who had volunteered for frontier labour in the hope of earning a ticket of leave, a chance at freedom paid for in sweat on the edge of a strange bay.

When the penal settlement closed, the causeway did not. It served a short-lived Catholic mission, then from 1850 a quarantine station, and it was here that the causeway witnessed one of colonial Queensland's grimmest arrivals. The emigrant ship Emigrant had sailed from Plymouth carrying assisted migrants and a stowaway: typhus. The fever killed people on the long voyage out, and when the vessel was put into quarantine at Dunwich, the dying continued. The ship's own surgeon, Dr George Mitchell, who had nursed his patients night and day across the ocean, fell ill and died here. So did Dr David Keith Ballow, the Moreton Bay health officer who came aboard to help and paid for it with his life. Both men lie buried together in the old cemetery at Dunwich, two doctors who walked into a plague to ease it.

From 1864 the site took on its longest and saddest role. The Dunwich Benevolent Asylum became a home for the colony's old, infirm, disabled and destitute, the people Victorian society had nowhere else to put. A government steamer called the Otter serviced it twice a week from Brisbane, and the causeway grew a timber jetty and a horse-drawn tramline to move supplies and people up into the settlement. The asylum ran until 1947, when overcrowding and decay finally closed it. Today the causeway survives almost intact beneath the modern barge ramp, its old tram tracks still glinting through gaps where the stone has worn away. It is easy to miss. But every arrival on Minjerribah still crosses, in a sense, the same red stones that convicts laid, the doorway to an island whose real history runs far older and far deeper than the wall itself.