The Portobello Peninsula, Quarantine Island and Goat Island almost close Otago Harbour to shipping. They are all part of a north-south aligned ridge of gently warped strata - an anticline - running from Portobello to Port Chalmers. Tectonic structures control much of the topography around the Otago Harbour. The Harbour is a synclinal depression situated between two large anticlinal ridges to the north and south. These are the Otago Peninsula and the Mihiwaka-North Head ridge.

The walk is best started either in Portobello village or at the aquarium car park. The start place you choose may depend on whether you want an ice-cream in the middle of the walk or at the end! The walk is described here in an anti-clockwise direction starting in the village, because the impact of the views from the top of the Portobello Peninsula is greater walking back to Portobello.

Turn left along the foreshore. Follow the strandline along the eastern side of the Peninsula. You are walking on small rocks broken up by weathering and erosion by the sea. A wide variety of shells are scattered underfoot. The fine-grained materials eroded from the cliffs form the remainder of the foreshore's sediments. In about 400 m a small shelly promontory is reached. Take some time to look at the shells in the deposits before continuing north past a derelict shack. Somewhere here Lamlash Bay becomes Portobello Bay.

Around another headland you will be surprised to come across a crib tucked tightly in under the cliff at one end of a short stretch of shelly beach. All around the crib is evidence for the landslips that scar the Portobello Peninsula. Continue north under the impressive almost-sheer cliffs to the stacks and islets on which oystercatchers, cormorants and gulls rest. Keep an eye out for falling rock - the cliffs high above deliver regular bombs into the sea. Gradually along this stretch, the foreshore changes to an intermittent rocky platform with seaweeds, mussels and the occasional oyster. On a calm day the clear water will permit you to observe the interplay of shallow marine life as the tides rise and ebb.

A short scramble over rocks around a little col in the cliffs brings you to another embayment and what appears to be the end of the easy going. However, continue by clambering over the base of the cliffs at the north end of the little bay and in twenty metres you will reach the not so secret sea cave. To enter the cave you will need to wade across the small embayment with luxuriant sealife. This entrance to the cave is narrow, but quite tall. On a bright day you won't need a torch, but put a hand in front of your temple to avoid banging your head on the very abrasive (!) volcanic rock. After about fifteen metres the narrow passage pops out into another, larger passage. On the right you can see out into the harbour and on the left is the intimidating maw of the second (larger) passage. Here, the water is quite deep and most visitors are kayakers.

When you have seen enough, retreat back to your entry point and return to the small embayment twenty metres back. Now climb straight up the steep hillside onto the cliff top for marvellous views across to Port Chalmers and down the harbour. This land is library endowment land and on the western side of the headland you will find a metalled walking track to the Portobello aquarium.

The aquarium sits on the end of the Portobello Peninsula across from Quarantine Island. The tide is restricted as it passes through the gap between the island and the Peninsula and at half-tide a tidal 'falls' may be seen, with a visible slope on the sea's surface. Quarantine Island was - as its name suggests - a place for the quarantining of new arrivals in the Otago colony. Derelict hulks are tied up to its wharf. More recently it has become a religious retreat. The aquarium is run by Otago University's Marine Science department for research, teaching and public education purposes. It is open to the public with restricted hours of opening.

Return to Portobello along the aquarium access track, enjoying the views across Latham Bay to Broad Bay and Portobello. Extensive mud flats are exposed at low tide, but a shallow channel hugs the Otago Peninsula and another is found around the western side of Quarantine Island. The pointed hill in front of you is Harbour Cone - a remnant of the volcanism that formed much of the Otago Peninsula.