Grimes is responsible for fully surveying the Hunter River in November 1801 with Francis Barrallier. In late 1802 Grimes commenced a survey of King Island and Port Phillip with Charles Robbins in . On 30 January 1803, whilst on his survey of Port Phillip he and his party landed at Frankston and met around thirty of the local inhabitants. On 2 February 1803 he discovered the mouth of the Yarra River. Next day Grimes ascended the river in a boat and explored what is now the Maribyrnong River for several miles. Returning to the Yarra it was explored for several miles but the boat was stopped by Dights Falls. The journal of another member of the party, James Flemming, has been preserved, and in it he several times refers to finding good soil. Although it was evidently a dry season Flemming, who was described by King as "very intelligent", thought from the appearance of the herbage that "there is not often so great a scarcity of water as at present". He suggested that the "most eligible place for a settlement I have seen is on the Freshwater (Yarra) River". A plaque at the site marks the event. Grimes returned to Sydney on 7 March 1803 and, in spite of Flemming's opinions, reported adversely against a settlement at Port Phillip.