Because of the large harbour area and narrow mouth between the Manukau Heads, tidal flow is rapid and a bar at the mouth makes navigating in or out of the harbour dangerous. New Zealand's most tragic shipwreck occurred on the bar in 1863 when HMS "Orpheus" ran aground in clear weather with a loss of 189 lives. For this reason, along with the harbour's shallowness, it is not Auckland's favoured port, and, with only one short wharf, the facilities at Onehunga are tiny compared to the other Ports of Auckland facilities on the Waitematā Harbour along the northeast of the isthmus.