Australian coach Tim Sheens believes the "NRL system" will face its biggest test over the next month when some of the next generation Kangaroos line up in the Four Nations tournament opener against Papua New Guinea this weekend.<br><br>Sheens believes the Four Nations will be a "big challenge" for the VB Kangaroos, with seven rookies named in the squad due to the unavailability of a number of the regular injured stars such as Greg Inglis, Johnathan Thurston and Justin Hodges.<br><br>The Four Nations event involving Australia, England, New Zealand and Papua New Guinea will be staged over three weeks with matches in Australia and New Zealand, with the Kangaroos opening their campaign against the Kumuls at Parramatta Stadium on Sunday.<br><br>"This will be a big test for the NRL system," Sheens said. "Everyone has always said that Australia’s next (players) and Australia’s next again can still beat the rest of the world. Well we have that scenario this year.<br><br>"England and New Zealand – with five games in New Zealand – are at a distinct advantage, so it’s going to be tough. And PNG will be enthusiastic. I don’t know what to expect from them."<br><br>Sheens said the Kumuls held the Australian Prime Minister’s XIII to 4-0 at half-time last month and "could have and should have" beaten England in the World Cup two years ago.<br><br>"Anything can happen," Sheens said. "They are a hard side to play against because I’ve got to motivate these guys (the Kangaroos) to keep their heads in this week; and because you don’t know what to expect from them (PNG).<br><br>"It’s our first game and we don’t want to be rusty and we don’t want to make too many mistakes and give the Kumuls a bit of a sniff. It’s a difficult game.<br><br>"I don’t think complacency will be there but there is rust in the team with a number of players not having played for awhile. Too many errors can make for a scrappy game.<br><br>"What is our best side is arguable. We have so many options."<br><br> For more information on the 2010 Four Nations go to <a href="http://www.rlfournations.com">www.rlfournations.com</a> and join us on <a href="http://www.facebook.com/fournations">Facebook</a>.