In March 1827 he was appointed police magistrate at Norfolk Plains and later at Campbell Town.
Dissatisfied with his prospects in Tasmania, Simpson joined the Port Phillip Association and in February 1836 offered his resignation to Lieutenant-governor Arthur who reluctantly reported to the Colonial Office that Simpson had "been infected with the Port Phillip mania."
As a member of the Port Phillip Association he had been allotted an area of land between the Werribee River and Station Peak, but held this for only a short time.
On 1 June 1836 the leading settlers of Port Phillip held the first public meeting at the township and by popular decision appointed Simpson as arbitrator in all disputes, except in matters relating to land with power to impose and collect fines.
At the same time Simpson was in the forefront of the business and cultural life of the Melbourne community.