William Cossar was a small 20 ton wooden New South Wales Colonial Government schooner that was wrecked in 1825.

[1] On 7 July 1817 a party of convicts stole William Cossar from Newcastle, New South Wales.

Nancy was sent to investigate and on 10 December 1818 William Cossar was found to be lying on a beach at the high tide mark.

The rigging, sails, gaffs, booms, bowsprit and rudder were missing, as was most of the copper sheathing.

On 11 March 1824, the ship capsized near Fort Macquarie, Sydney Harbour, in a squall, and three men drowned.