South Africa has woken up, Australia, England and Canada are growing threats, and Fiji always present a stern test.
Since the world series began in 1999-2000, New Zealand have won the crown 12 times in 15 seasons.
Forbes, 32, has said for the past few years that it was just a matter of time until the chasing pack caught up. That time has arrived.
South Africa have won two of the first three tournaments this season and lead the overall standings ahead of Fiji with New Zealand in third.
New Zealand must finish in the top four this year to guarantee qualification for next year's Olympics.
Forbes said continuity was the key to the rise of the likes of South Africa, who lost to New Zealand in the final of the Wellington leg last year.
"A lot of the teams we see now have a core group and that's probably been the difference," Forbes said.
"You can bring all the guys in from 15s and guys that have been in and out of the team but the teams that are benefiting are the teams that have a core group and they're working the team around that."
New Zealand coach Gordon Tietjens has been able to call upon the likes of Forbes, Tim Mikkelson, Scott Curry and Sherwin Stowers in recent years, which has helped shape the squad.
Since sevens was accepted into the Olympics, people in New Zealand have been speculating about which All Blacks might be included for next year's event.
Names such as Sonny Bill Williams, Ben Smith and Julian Savea keep cropping up.
Tietjens has said any converts would have to play four tournaments on the world series circuit next season and Forbes said it wouldn't be an easy transition.
"The key thing is going to be having that special group of sevens players and then bringing guys in from 15s or other codes to try to complete the package."
The Wellington tournament, which runs today and tomorrow, is the fourth of nine on the international circuit.
New Zealand start with a Pool B outing against Canada at 2.08pm. They will also meet Papua New Guinea and England in the group stages.
5 things about the sevens
1. Seven titles
In 15 previous tournaments in Wellington, New Zealand have claimed seven titles. They beat South Africa 21-0 in last year's final.
2. More winners
Other teams to win in Wellington include: Fiji (three times), England (twice), Samoa, Australia and South Africa.
3. Tries galore
Kenyan stalwart Collins Injera, who will play in Wellington this year, has the third-most tries in the history of the world series with 189. Injera has the most tries among active players; retired Argentinian Santiago Gomez Cora leads the pack with 230.
4. Top tryscorer
After three tournaments in this season's world series, South African Seabelo Senatla leads the charge for the most tries with 20.
5. Newcomers
New Zealand will field four world series debutants this weekend: Jack Goodhue, Beaudein Waaka, Dylan Collier and Rieko Ioane.