She was completed too late to take part in the American War of Independence, and instead spent several years laid up in ordinary, before finally entering service as a store and convict transport to Australia, under Lieutenant Edward Riou.

Eventually through continuous work he and the remaining crew were able to navigate the ship, by now reduced to little more than a raft, back to the Cape, a nine-week voyage described as "almost unparalleled".

[1] After five years spent laid up she was fitted out at Woolwich in 1789 to serve as a store and convict transport, commissioning under Lieutenant Edward Riou in April.

[1] Riou was tasked with delivering the stores consisting of seeds, plants, farm machinery and livestock, with a total value of some £70,000 (equivalent to about £10,870,000 in 2023), and convicts to the British settlement at Botany Bay.

[3][5] With over 300 people aboard his ship, Riou left Spithead on 8 September, and had an uneventful voyage to the Cape of Good Hope, where he arrived on 24 November and loaded more livestock and plants.

[3][5] After completing his re-provisioning Riou sailed from the Cape in mid-December, and picking up the westerlies began the second leg of his voyage to New South Wales.

[3] On 24 December, twelve days and 1,300 statute miles (2100 km) after his departure from the Cape, a large iceberg was spotted at 44°S 41°E / 44°S 41°E / -44; 41, and Riou decided to use the ice to replenish his stocks of fresh water, that were quickly being depleted by the need to supply the plants and animals he was transporting.

Riou ordered the stores, guns and livestock to be thrown overboard in an attempt to lighten the ship, but was injured when his hand was crushed by a falling cask while trying to clear the bread-room.

[6] Riou at last agreed to this, well aware that there were not enough boats for everyone, and announced "As for me, I have determined to remain in the ship, and shall endeavour to make my presence useful as long as there is any occasion for it.

"[7] While the boats were prepared, Riou wrote a letter to the Secretary to the Admiralty; Sir,If any part of the officers or crew of the Guardian should ever survive to get home, I have only to say their conduct after the fatal stroke against an island of ice was admirable and wonderful in everything that related to their duties considered either as private men or on his Majesty's Service.As there seems no possibility of my remaining many hours in this world, I beg leave to recommend to the consideration of the Admiralty a sister who if my conduct or service should be found deserving any memory their favour might be shown to her together with a widowed mother.I am Sir remaining with great respectYour ever Obedt & humble servt,E.

Realising that this was providing extra buoyancy, Riou had the gun deck hatches sealed and caulked, while another sail was sent under the hull to control the flooding.

Having now created a substitute hull out of his deck, Riou raised what little sail he could and began the long journey back to land, with the pumps being continuously manned.