Towards the end of 1790 Grimes was appointed deputy surveyor of roads in New South Wales and allocated work at Norfolk Island.

At Norfolk Island he was employed correcting a previous survey which had been made without proper instruments, and he also undertook some of the administrative work.

In February 1795 he sailed north in the Francis and spent approximately one week at Port Stephens and reported unfavourably on the locality.

On 30 January 1803, whilst on his survey of Port Phillip he and his party landed at Frankston where he met around thirty Indigenous people.

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".

In March 1807 he was sent to Port Dalrymple, and using the advice of Thomas Laycock, who a month earlier had become the first European to traverse the interior of Tasmania, Grimes made a survey of the district and examined the route to Hobart.

Grimes returned at the end of the year, and became involved in the deposition of Governor William Bligh in the Rum Rebellion on 26 January 1808.

Grimes realised, however, that his lack of legal training made it difficult for him to act effectively in his judicial position and he resigned on 5 April 1808.