The 25 year-old died alongside his father, Richard Cousins, who was chief executive of catering company Compass Group, and his brother, Edward.
Cousins’ fiancé Emma Bowden, her daughter Heather, and the plane’s pilot Gareth Morgan also died in the crash at Hawkesbury River, about 30 miles from Sydney.
William Cousins worked for Open Britain, the group that campaigns against a so-called ‘hard Brexit’. In a statement, the organisation said it "has lost part of its soul today".
Roland Rudd, chairman of PR agency Finsbury who also chairs Open Britain, said: "All of us at Open Britain are devastated by the tragic loss of Will and his family.
"Will was an extraordinary young man who was passionate about what he did; who took deep pride in his work; and came into the office every day with enthusiasm, energy and determination.
"Will saw his role at Open Britain as more than a job. For him, it was a cause akin to a crusade to stop Britain crashing out of the EU with no deal or a bad deal.
"Will was a great team player who loved the camaraderie of our young team and developed close relationships with all his colleagues. He will be missed beyond words."
Open Britain board member Lord Mandelson said: "Will was a tremendous talent and a really great guy and team player. He was indefatigable in his work for Open Britain and we will miss him and honour him by continuing his work in the campaign."
Tributes have also been paid on social media:
I was absolutely devastated to learn this morning of the death of Will Cousins, head of press for @Open_Britain, and his family in a tragic plane crash in Australia. 1/3— Chuka Umunna (@ChukaUmunna) January 1, 2018
I got to know Will very well, first through the Stronger IN campaign, then afterwards at Open Britain - where he has played a crucial role - and through @LeaveWatch_ which he helped establish. 2/3— Chuka Umunna (@ChukaUmunna) January 1, 2018
Will was an absolute pleasure to work with, utterly dedicated to the cause, dynamic &full of enthusiasm. He went about things with great heart and humour. He had his whole life ahead of him and one cannot compute that it has been taken from him. We'll all miss him so much. 3/3— Chuka Umunna (@ChukaUmunna) January 1, 2018
Will was a lovely young man (so calm & patient). He came from a remarkable family. Genuine loss & tragedy. Thoughts w all who knew them. https://t.co/7moa1Kync9— Anna Soubry MP (@Anna_Soubry) January 1, 2018
Devasted to hear about the tragic passing of my @Open_Britain colleague Will Cousins. He was a talented, smart, fun and brilliant individual with so much more left to give our country, such a sad loss for us all. May he rest in peace. #Gonetoosoon https://t.co/eSl8sfMKhu— June Sarpong MBE (@junesarpong) January 1, 2018
A terrible tragedy - Will was clearly a talented and passionate young man with so much to offer the world. Thoughts are very much with all his family and friends. https://t.co/LzVzSwNufk— stellacreasy (@stellacreasy) January 1, 2018