The 29-year-old ball-handling back-rower will arrive in time for next season on a two-year contract and is seen as a replacement for another Australian forward, Marc Glanville, who is retiring after two successful seasons with the club.
"David is a current State of Origin player and one of the best forwards in Australia," said the Leeds chief executive, Gary Hetherington. "He is very keen to be coming to England and it is significant that our competition has attracted another top performer. We are losing a lot of experience with Marc retiring, but David is a very, very mobile forward with a big work rate and good skills. He will be very well suited to the English game."
Barnhill previously played for Canberra and St George, and is bidding to reach his sixth Grand Final in Australia before embarking on his new career in Yorkshire.
Hetherington says that Leeds' overseas recruitment for next season is now complete. They announced the signing last week of Paul Bell, the Melbourne centre who will replace Brad Godden, who has also decided to retire after playing a major part in Leeds' recent success.
"He was thinking of having another year, but his wife is expecting their third child and he has decided to call it a day," Hetherington said. The club does not intend to try to replace Richie Blackmore, the New Zealand Test centre who has missed almost all this season with a groin injury, on their overseas quota. Blackmore has another year to run on his contract.
"He has had an operation and we are hoping that he is going to be alright," said Hetherington, who has had an offer for Keith Senior rejected by Sheffield Eagles. "That is probably the end of our recruiting, because we will only be allowed 20 players on contracts of over pounds 20,000 next season."
Leeds will have Adrian Morley and Anthony Farrell back from suspension for Friday night's match at Bradford which could see them go above St Helens into second place in the JJB Super League - a position that would give them a home draw against Saints in the play-offs.
The Leeds coach, Graham Murray, hopes to have Marcus St Hilaire fit again, but there are doubts about Ryan Sheridan's knee, Jamie Mathiou is still unfit and Terry Newton remains suspended.Reuse content