From 6 June to 16 June 1789, John White, John Hunter and David Collins were amongst a large party that accompanied Governor Phillip on a second expedition to Broken Bay, in the course of which an extensive river was discovered.  Phillip named it the Hawkesbury River.  At the end of the month the same party traced the course of the river upstream to the present Richmond area.

Watkin Tench, Captain of Marines, was unable to be part of either expeditions:

At this period, I was unluckily invested with the command of the outpost at Rose Hill, which prevented me from being in the list of discovers of the Hawkesbury.

Tench engaged in several expeditions in NSW to explore the surrendering country side. He discovered a river that Phillip named Nepean after his friend Evan Nepean, who was an Under Secretary in the Home Office.

Stimulated, however, by a desire of acquiring a further knowledge of the country, on the 26^th instant accompanied by Mr Arndell, assistant surgeon of the settlement, Mr Lowes, surgeon’s mate of the Sirius, two marines and a convict, I left the redoubt at daybreak, pointing our march to a hill five miles in a westerly or inland direction, which commands a view of the great chain of mountains called Carmarthen Hills, extending from north to south farther than the eye can reach.

We continued to march all day through a country untrodden before by an European foot.  Save that a melancholy crow now and then flew croaking overhead, or a kangaroo was seen to bound at a distance, the picture of solitude was complete and undisturbed.

At four o’clock in the afternoon we halted near a small pond of water where we took up our residence for the night, lighted a fire, and prepared to cook our supper: that was to broil over a couple of ramrods a few slices of salt pork and a crow which we had shot.

At daylight we renewed our peregrination and in an hour after we found ourselves 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.

Having remained out three days, we returned to our quarters at Rose Hill with the pleasing intelligence of our discovery.  To this river the governor gave the name of Nepean. 

Reference, Watkin Tench, 1788 A Complete Account of the Settlement of Port Jackson.

         The Nepean River is a major perennial river, located in the south-west and west of Sydney, New South Wales.  The River and its associated mouth, the Hawkesbury River, almost encircles the metropolitan region of Sydney. It took about three years after discovery to realise that the Nepean flowed into the Hawkesbury. (Wikipedia)

1789 – A timeline in the lives of the First Fleeters

27 March	Six marines hanged for stealing provisions from the public store.
15 April	Smallpox discovered among the Aboriginal people.
21 April	The Sirius damaged by a storm off the southern coast of Tasmania.
8 May	The Sirius arrived in Port Jackson with relief supplies.
18 May	Arabanoo died of smallpox.
2 June	George Farquar’s play The Recruiting Officer performed by the convicts.
6 June	Governor Phillip led an expedition to Broken Bay to trace the Hawkesbury River.
26 June	Watkin Tench led an expedition west from Rose Hill and discovered the Nepean River.
8 August	The night-watch was formed in response to an increasing number of thefts.
1 November	With provisions running short, the men’s rations was reduced by one third.
21 November	James Ruse, whose term had expired, was settled on two acres at Rose Hill.
25 November	Bennelong and Colbee captured.
12 December	Colbee escaped from the settlement.
23 December	HMS Guardian carrying relief supplies from England, struck an iceburg.

Reference: Jack Egan, Buried Alive, Sydney 1788-82, Eyewitness account of the making of a nation.