This trip, inspired by Tench's ardent love of discovery, became a noteworthy one in the annals of New South Wales.

It was made during the month that witnessed the discovery of the Hawkesbury River.

On the second day after his party left Rose Hill, they found themselves early in the morning on "the banks of a river, nearly as broad as the Thames at Putney, and apparently of great depth, the current running very slowly in a northerly direction." This river, at first known as the Tench, was afterwards named the Nepean by Phillip, when its identity as a tributary of the Hawkesbury had been confirmed.

Two other slight excursions were made by Tench in company with Lieutenant Dawes, who was in charge of the Observatory, and ex-surgeon Worgan.

In May, 1791, Tench and Dawes started from Rose Hill and confirmed the supposition that the Nepean was an affluent of the Hawkesbury, a matter over which there had been some doubt since its first discovery by Tench.