Mansion House bay is a very popular destination for travellers to New Zealand and Locals, as it provides the historic Mansion House to look through some New Zealand History, a cafe, a grassy area to picnic, run and play, sheltered swimming, a number of walking tracks to other bays, and looks outs.
You can bring your boat, or catch a ferry or water taxi from Sandspit, it your really keen you could even kayak from any one of the local land beaches.
For the locals it is good sheltered anchoring with fair holding as the bay wraps around headland with high trees.
Kawau Island has a fascinating history. Kawauisland.org provides a lovely summary from pre-1840 when the island was occupied by Maori tribes, the exchange of the entire island for a few muskets, the sale to Sir George Gray who built the Mansion, through to now where Mansion House is run by the Department of Conservation for all of us to enjoy.