Sandy Beach sits at Te Rāpaki-o-Te Rakiwhakaputa, the Ngāi Tahu settlement of Rapaki on the northwestern shore of Lyttelton Harbour beneath the Port Hills below Cashmere, and is rated 4.3 from 16 Google reviews.
Harbour beaches like those dotted around Banks Peninsula trade surf for shelter: the water is typically calm and gentle, which suits paddlers, paddleboarders and children, but the tide shapes the day far more than at an open-coast beach, with sand and shallows at high water and mud or shingle exposed as it drops. Check a tide chart before setting out, pack water, sun protection and sturdy shoes, and expect minimal facilities at the smaller bays. Locals tend to time their swims for the top of the tide and keep an eye on the nor'wester that pours over the Port Hills.
Sandy Beach makes a quiet, sheltered spot for a harbour swim, reached by road or on foot down the Rapaki Track from Cashmere.