Middle Harbour is a tide-dominated valley estuary forming part of Sydney Harbour in New South Wales, Australia. It extends inland from the main harbour, narrowing progressively upstream into Middle Harbour Creek, and is flanked by Hawkesbury sandstone outcrops, dense eucalypt forest dominated by angophoras, blackbutts, banksias, she-oaks, and native grass trees. The waterway transitions from open harbour with yacht moorings and marina activity near The Spit to increasingly quiet, glassy water upstream, where paddlers can find near-complete solitude despite being within suburban Sydney.

The primary access point for paddlers is The Spit Bridge in Mosman, where Sydney Harbour Kayaks operates a rental facility at Smiths Boat Shed, 81 Parriwi Road. An additional access point is the boat ramp at Tunks Park in Cammeray. The Spit Bridge opens at scheduled intervals to allow larger vessels through, though low-profile kayaks can pass beneath without waiting for the bridge to lift.

Notable features along the route include white sandy beaches, healthy mangrove systems, historic shipwrecks, a lighthouse, and a decommissioned explosives depot. Wildlife commonly observed includes herons, cormorants, and sea eagles, along with marine life such as moon jellyfish, saltwater fish, and occasional dolphins, seals, and little penguins. The mangrove systems in particular represent a functioning ecological environment that guided tours specifically highlight.

Boat traffic is a relevant consideration, particularly near the marina and moorings around The Spit, where large yachts and working vessels operate. Upstream past Roseville Bridge, traffic diminishes significantly. Wind can be a factor on the more open sections of the harbour, as noted by paddlers familiar with the area.

Kayak and surf ski rentals are available at The Spit Bridge location from Sydney Harbour Kayaks, with options ranging from hourly to full-day hire, including single and double kayaks and surf skis from brands such as Mirage Sea Kayaks, Fenn, and Carbonology. Guided eco tours and self-guided options are offered, including a mangrove-focused tour. The operator also provides free introductory kayaking courses. Slipway and haul-out facilities for larger vessels are available at various points around the harbour.