Approximately 15 km in length from its mouth to Teddington, the harbour was formed from a series of ancient volcanic eruptions that created a caldera, the steep sides of which form the Port Hills on its northern shore.
The harbour's main population centre is Lyttelton, which serves the city of Christchurch, linked with Christchurch by the single-track Lyttelton rail tunnel (opened 1867), a two lane road tunnel (opened 1964) and two roads over the Port Hills. Diamond Harbour lies to the south and the Māori village of Rāpaki to the west. At the head of the harbour is the settlement of Governors Bay. The reserve of Quail Island is near the harbour head and Ripapa Island is just off its south shore at the entrance to Purau Bay.
The harbour provides access to a busy commercial port at Lyttelton which today includes a petroleum storage facility and a modern container and cargo terminal.