Bass had brought with him on the Reliance a small boat with an 8-foot (2.4 m) keel and 5-foot (1.5 m) beam, which he called the Tom Thumb on account of its size.

In October 1795 Bass and Flinders, accompanied by William Martin sailed the Tom Thumb out of Port Jackson to Botany Bay and explored the Georges River further upstream than had been done previously by the colonists.

Later that year Bass discovered good land near Prospect Hill, found lost cattle brought out with the First Fleet, and failed in an attempt to cross the Blue Mountains.

His belief that a strait separated the mainland from Van Diemen's Land (now Tasmania) was backed up by his astute observation of the rapid tide and the long south-western swell at Wilson's Promontory.

When the two returned to Sydney, Flinders recommended to Governor John Hunter that the passage between Van Diemen's Land and the mainland be called Bass Strait.

"This was no more than a just tribute to my worthy friend and companion," Flinders wrote, "for the extreme dangers and fatigues he had undergone, in first entering it in a whaleboat, and to the correct judgement he had formed, from various indications, of the existence of a wide opening between Van Diemen's Land and New South Wales."

"In this voyage of fourteen weeks I collected those few plants upon Van Diemen's Land which had not been familiar to me in New South Wales," he wrote to Banks, "and have done myself the honour of submitting them to your inspection."

[10] Bass and a syndicate of friends had invested some £10,000 in the copper-sheathed brig Venus, and a cargo of general goods to transport and sell in Port Jackson.

(Among his influential friends and key business associates in the Antipodes was the principal surgeon of the satellite British colony on Norfolk Island, Thomas Jamison, who was subsequently appointed Surgeon-General of New South Wales.)

It seems, as Flinders' biographer Ernest Scott observed, that Bass's natural modesty meant he felt no need to say "discovered by me" or "named after me".

With his partner Charles Bishop, Bass sailed from Sydney in the Venus for Dusky Sound in New Zealand where they spent 14 days stripping iron from the wreck of Captain Brampton's old ship the Endeavour.

[12] In January 1803, Bass applied to King for a fishing monopoly extending from a line bisecting the lower South Island of New Zealand from Dusky Sound to Otago Harbour – now the site of the city of Dunedin – and including all the lands and seas to the south, notably the Antipodes Islands, probably on the basis of information from his brother-in-law Waterhouse, the discoverer of the Antipodes archipelago.

Port Jackson was described by some 19th-century historians as a base for such smuggling[14] Britain's strained relationship with Spain at that time meant British authorities were unconcerned.

As many months passed with no word of his arrival, Governor King and Bass's friends in Sydney were forced to accept that he had met some misfortune.

Another factor against the South American story is that all British prisoners held by the Spanish in Chile and Peru were freed in 1808 and returned to Europe.