New Zealand is a sovereign state with a democratic parliamentary government based on the Westminster system. Its constitutional history dates back to the signing of the Treaty of Waitangi in 1840. Its government is formed from a democratically elected House of Representatives. The Government advises our head of State, who is the Queen. By convention, the Queen, the source of all executive legal authority in New Zealand, acts on the advice of the Government in all but the most exceptional circumstances. This system is known as a constitutional monarchy.