Already a host of high-profile stars are expected to depart the Super Rugby stage after the global tournament in England and Wales in September and October.

Richie McCaw, Dan Carter, Ma'a Nonu, Keven Mealamu, Tony Woodcock, Bismarck du Plessis, Victor Matfield, Jean de Villiers, Duane Vermeulen and Adam Ashley-Cooper are among frontline test campaigners who will either retire or chase a golden years contract in Europe or Japan. More will follow in coming months.

When combined with the host of fringe All Blacks and polished Super Rugby campaigners snapped up by French and English clubs, the 2016 playing resources for Sanzar's premier franchise competition are looking the thinnest in its two-decade history.

The problems are amplified by the Rio Olympics where the likes of Sonny Bill Williams, Liam Messam and possibly Julian Savea will make only minor appearances in the competition due to sevens commitments.

With a sixth South African team, the new Argentine and Japan-based franchises also entering the competition in 2016, there are genuine fears the playing talent will be spread too thinly and the competition may fall flat.

The game's faltering position in Australia, where it is slipping further behind league, football, and AFL, is also causing headaches.

The competition's financial bottom line is another worry. While Super Rugby can justifiably claim to be world rugby's premier franchise competition, it can't compete with the financial backing in France, England or Japan.

Eight of rugby's top 10 highest earners reside in the bosom of wealthy French club owners who are willing to pay up to $2 million per season, with the other two plying their trade in England. There are 42 international players among the top four Japanese club teams.

Top players here have the ability to double, even triple, their earning potential with a move abroad that also includes less travel - an appealing lure for players with children.

"We don't have the budgets that some of the French clubs have because there's no logic to it," New Zealand Rugby chief executive Steve Tew admitted. "They are owned by people with super yachts. We've got income that we can afford to allocate to retaining our top professional players and we won't spend more than we can afford. We can't close the gap."

The imminent loss to Europe of Tom Marshall, 24, Luke Braid, 26, Ben Tameifuna, 23, and Frank Halai, 26, underline how players are now also leaving at a younger age to cash in on overseas opportunities.

"These guys now, this is what they do for a vocation; it's how they earn their living," All Blacks selector Grant Fox said. "If some of them think getting through to the All Blacks is a challenge they might go sooner than we would like, rather than stay and fight for the jersey. But when one goes it's an opportunity for someone else. We've got a pretty good factory coming through."

Traditionally, New Zealand's seemingly endless production line of talent stayed here in pursuit of the black jersey. For some that remains the case but there are clear signs those days are numbered. Retaining top players, and coaches, has never been more difficult. Eventually it could create a competition full of 18- to 20-year-olds with limited experience.

"The reality is the World Cup becomes a pretty important juncture in people's decision-making processes," Tew said. "We can't control that. Our contracting team are ahead of the game.

"No-one has slipped through the cracks, it's just they have options and they're exercising them. I don't think it's any worse but we won't know until we get through this cycle."

Australia has pressing concerns. Ashley-Cooper, Scott Higginbotham, Sekope Kepu, Nic White and James Horwill are all set to leave and the game there will be desperate to retain Will Genia, Quade Cooper, Kurtley Beale and Israel Folau who come off contract at the end of this year.

South Africa bit the bullet and now selects players from overseas clubs. While New Zealand persists with its staunch stance, pressure is mounting on Australia to follow the Springboks' lead.