Boyd Cordner scored the only try as the Kangaroos were forced to defend for their lives in a tense game.
Full-back Widdop said: “We have sacrificed so much and worked so hard and we are certainly very disappointed but proud of the boys and the effort they put in.
“We created a fair few breaks but we just couldn’t finish and it was our execution and in the end they were too good.
“We have certainly come a long way in the last couple of years with Wayne (Bennett) at the helm, from this year to last year we have certainly improved a lot and we are certainly going to keep improving.
“So I am so proud of the boys for the effort they have put in over the last seven weeks. It is very disappointing but we will have to move in from this.”
Australia’s Cameron Smith, who admits he could retire, knew his side had been in an epic contest.
“It is amazing to finally get a World Cup victory on home soil, it has been quite a long time and a lot of hard work has gone into it,” he said.
“It was one of the toughest football games I have played in my entire career so it is a really pleasing victory for the boys.
“It has been a fairly hard road over the last two years, we wanted to put ourselves back to number one, we have got back-to-back World Cup victories now and I am very happy to have the Kangaroos as the best team in the world.
“We spoke in the very first meeting of this campaign about defence and that it wins matches and tournaments as well. We conceded 16 points over six games, that is a pretty remarkable effort, but especially tonight. I am really happy for all the boys.”