Taken from a previous answer:

The USA has Thanksgiving. Australia's equivalent is...erm...what Grace Karskens has dubbed 'The Foundational Orgy'.

For context, the first ships full of British convicts - the First Fleet - landed in Australia on January 26th, 1788. However, the convicts on one of the ships in the First Fleet, the Lady Penrhyn, were not allowed to land at Sydney Cove until February 6th. The main reason for this is that the convicts on the Lady Penrhyn were female, unlike the rest of the convicts (and unlike the military staff running the colony). Huts and tents needed to be constructed first before women would be allowed on shore (or something like that).

The main evidence for the 'Foundational Orgy' is an entry in the journal of the Lady Penrhyn's surgeon, Arthur Bowes-Smyth for the 6th of February.

6th. At 5 o'Clock this morng. all things were got in order for landing the whole of the women & 3 of the Ships Long Boats came alongside us to receive them: previous to their quitting the Ship a strict search was made to try if any of the many things wh. they had stolen on board cd. be found, but their Artifice eluded the most strict search & abt. 6 O'Clock p.m. we had the long wish'd for pleasure of seeing the last of them leave the Ship -- They were dress'd in general very clean & some few amongst them might be sd. to be well dress'd. The Men Convicts got to them very soon after they landed, & it is beyond my abilities to give a just discription of the Scene of Debauchery & Riot that ensued during the night --

They had not been landed more than an hour before they had all got their Tents pitched or anything in order to receive them, but there came on the most violent storm of thunder, lighteng. & rain I ever saw. The lighteng. was incessant during the whole night & I never heard it rain faster -- Abt. 12 o'Clock in the night one severe flash of Lightg. struck a very large tree in the centre of the Camp under wh. some places were constructed to keep the Sheep & Hogs in: it split the tree from top to bottom; kill'd 5 Sheep belonging to Major Ross & a pig of one of the Lieuts. -- The severity of the Lighteng. this & the 2 preceeding nights leaves no room to doubt but many of the trees wh. appear burnt up to the tops of them were the Effect of Lightening --

The Sailors in our Ship requested to have some Grog to make merry wt. upon the Women quitting the Ship indeed the Capt. himself had no small reason to rejoice upon their being all safely landed & given into the Care of the Governor, as he was under the penalty of 40£ for every Convict that was missing -- for wh. reason he comply'd wt. the Sailor's request, & abt. the time they began to be elevated, the Tempest came on -- The Scene wh. presented itself at this time & during the greater part of the night, beggars every discription; some swearing, others quarrelling others singing, not in the least regarding the Tempest, tho' so violent that the thunder shook the Ship exceeded anything I ever before had a conception of. I never before experienced so uncomfortable a night expectg. every moment the Ship wd. be struck wt. the Lighteng. -- The Sailors almost all drunk & incapable of rendering much assistance had an accident happen'd & the heat was almost suffocating.

And specifically, the idea of the Foundational Orgy comes specifically from the line

The Men Convicts got to them very soon after they landed, & it is beyond my abilities to give a just discription of the Scene of Debauchery & Riot that ensued during the night --

Because, of course, while it might have been beyond Bowes-Smyth's abilities to give a just description of that debauchery, historians of the 20th century were very happy to fill in the gaps. See Robert Hughes in The Fatal Shore, who lets imagination run a little riot, and whose prose about this isn't out-of-place amongst 20th century historiography on the topic:

It was a squally day, and thunderheads were piled up in livid cliffs above the Pacific; as dusk fell, the weather burst. Tents blew away; within minutes the whole encampment was a rain-lashed bog. The women floundered to and fro, draggled as muddy chickens under a pump, pursued by male convicts intent on raping them. One lightning bolt split a tree in the middle of the camp and killed several sheep and a pig beneath it. Meanwhile, most of the sailors on Lady Penrhyn applied to her master, Captain William Sever, for an extra ration of rum “to make merry with upon the women quitting the ship.” Out came the pannikins, down went the rum, and before long the drunken tars went off to join the convicts in pursuit of the women, so that, Bowes remarked, “it is beyond my abilities to give a just description of the scene of debauchery and riot that ensued during the night.” It was the first bush party in Australia, with “some swearing, others quarrelling, others singing—not in the least regarding the tempest, tho’ so violent that the thunder shook the ship exceeding anything I ever before had a conception of.” And as the couples rutted between the rocks, guts burning from the harsh Brazilian aguardiente, their clothes slimy with red clay, the sexual history of colonial Australia may fairly be said to have begun.

Karskens, of course, invoked the idea of the Foundational Orgy in order to bury it. Her book, The Colony is fairly clearly an attempt to revise our understanding away from the impression of a brutal, dead-end hell that you get from Robert Hughes' The Fatal Shore. Hughes' book is wrapped up in a grim - almost gothic - view of human nature. People, in The Fatal Shore, react to events exactly as poorly as you might expect if you have a dim view of humanity. So of course, when the men finally have access to women after what must have seemed like an eternity, they gave in to every base desire. Perhaps the women do too.

The Sydney colony, in Karskens' eyes, however, was not all wine and roses, but...there was some wine and some roses? Hughes portrays the colony as basically Deadwood Down Under, a brutal and only nominally lawful, masculine-dominated testosterone-filled place that was no place for poor women - 'muddy chickens under a pump' who were 'pursued by male convicts intent on raping them,' in Hughes' words. Karskens, perhaps unsurprisingly, given she was writing The Colony this century, is much more interested in the experiences of women in the early Australian colonies than Hughes is )her book is about the colony more generally; she is just correcting the balance). What she finds going through those experiences is that the Colony was generally a kinder place to women than you'd imagine from The Fatal Shore.

It's therefore squarely within Karsken's interests to debunk the Foundational Orgy. For Karskens, it never happened. For Karskens, the reason why Bowes-Smyth was unable to describe the debauchery was because he was not there, being stuck on the Lady Penrhyn in an organisational role - she argues he never went on shore. Karskens argues that the existence of debauchery and riot was, therefore, largely in Bowes-Smyth's (and, later, Hughes') imagination. There were no rutting between the rocks. The sailors on the Lady Penrhyn making merry stayed on the ship, glad that their charges, who they largely despised, were no longer their charges.