On 25 May 1768, the Admiralty commissioned Cook to command a scientific voyage to the Pacific Ocean. The purpose of the voyage was to observe and record the 1769 transit of Venus across the Sun which, when combined with observations from other places, would help to determine the distance of the Earth from the Sun.
“The expedition sailed aboard HMS Endeavour, departing England on 26 August 1768. Cook and his crew rounded Cape Horn and continued westward across the Pacific, arriving at Tahiti on 13 April 1769, where the observations of the Venus Transit were made. However, the result of the observations was not as conclusive or accurate as had been hoped. Once the observations were completed, Cook opened the sealed
orders, which were additional instructions from the Admiralty for the second part of his voyage: to search the south Pacific for signs of the postulated rich southern continent of Terra Australis. Cook then sailed to New Zealand, taking with him Tupaia, an exceptionally accomplished Tahitian aristocrat and priest, who helped guide him through the Polynesian islands, and mapped the complete coastline, making only some minor errors. He then voyaged west, reaching the southeastern coast of Australia on 19 April 1770, and in doing so his expedition became the first recorded Europeans to have encountered its eastern coastline. – Wikipedia
The expedition landed in Botany Bay (Sydney) on 28 April 1770.Visit Event Site