Jonah Lomu during the 1999 Rugby World Cup semifinal between the All Blacks and France.

New Zealand boasts a unrivalled history of catapulting top rugby sevens performers into the test arena – though that pathway is becoming closed off, according to DJ Forbes, the legendary former Sevens skipper.

Sevens has helped propel the careers of a multitude of New Zealand greats, including Jonah Lomu, Joe Rokocoko, Julian Savea, Mils Muliaina, and Christian Cullen. Superstar wing Rieko Ioane is the latest is a lengthy list of All Blacks who first represented New Zealand on the international sevens circuit.

Indeed, in the All Blacks' narrow 16-15 victory over England at Twickenham in November there was an incredible statistic that was cheered by supporters of the abridged version of the game: five of the seven starting backs – Ioane, Ben Smith, Jack Goodhue, Sonny Bill Williams, and Beauden Barrett – had all represented their country's sevens team.

However, Forbes, who retired in 2017 after a decade of captaincy, believes the two strands of rugby union have become so different that it is now impossible for players to interchange between the formats successfully.

ROSS BROWN Julian Savea training with the NZ sevens team at Mt Maunganui.

"Sevens has been used as a launchpad for players, historically, to expose their talents so they can reach the next level," Forbes tells Stuff, on the eve of the HSBC New Zealand Sevens in Hamilton this weekend.

"Now the two sports are going their separate ways."

The inclusion of sevens at the Olympics, in Rio de Janeiro three years ago, triggered an explosion of global interest and has accelerated its progress. Because of its elevated status, the chance of going for gold, and with more money in the game, players are having to make a definitive choice between concentrating on either sevens or 15-a-side rugby.

"There is still an opportunity to use the sevens circuit as a bit of a testing ground, but the way it is going it is turning into a specialist sport," continues Forbes, 36.

"With it being a full-time gig now, players have to make a decision to follow their sevens path, or the 15s path. It will be interesting to see how it develops."

MARTIN HUNTER Christian Cullen looks to off load the ball during training for NZ Sevens team .

Forbes hung up his boots after a record 89 world series competitions, having played 512 games, scored 153 tries – he is currently in 12th place in the all-time list – and chalked up 26 tournament victories. In addition, he skippered his country to glory at a Commonwealth Games in 2010 and, three years later, the World Cup.

"It has been amazing to watch the growth of sevens around the world. It has developed so much since I made my debut in 2006," he adds. "Being an Olympic sport, countries can apply for grants and that has changed the game. It provides the opportunity for smaller nations, who might not previously have had the resources or manpower, to get involved with rugby."

LAURENCE GRIFFITHS/GETTY IMAGES Beauden Barrett was in the sevens squad.

Almost exactly three years ago Sonny Bill Williams made his sevens debut at the Wellington tournament, having set his sights on selection for the Rio Games. His experience in that competition illustrated the gulf between the codes, even back then.

There was one signature high-risk, out-of-the-tackle offload that led to the winning try in the final against South Africa, which ended 24-21; but there were two similar attempts that were costly errors. Moreover, Williams was obviously struggling to match the fitness of his new teammates and lagged behind play despite only being afforded limited game time.

JUAN MANUELSERRANO ARCE/GETTY D J Forbes scores a try on day one of the 2017 Hong Kong Sevens.

That his initial performance was disappointing surprised many, though it didn't shock those in the know. "I left my lungs out there on the pitch," Williams admitted in a pitch-side interview.

The New Zealand team has failed to win on home soil since that 2016 victory. Last year, in the inaugural competition in Hamilton, eventual winners Fiji toppled them in the semi-final.

Hamilton is the third stop in the HSBC World Rugby Sevens Series this season, and New Zealand are currently second in the overall rankings, one point behind America. On Saturday, in Pool D, they face Japan, Canada, and Spain, and will be confident of securing a quarter-final berth on Sunday.

Oliver Pickup is a London-based, multi-award-winning freelance journalist. He specialises in sport – particularly rugby union – as well as technology. He can be found on Twitter at @OliverPickup.