Warburton wrote in British newspaper The Times that the top northern hemisphere sides have closed the gap that has long separated the world number one.
“Six months ago, I would probably have put my house on New Zealand being World Cup winners. I wouldn’t put that bet on now,” Warburton said.
“It’s going to be some World Cup next year in Japan.”
Warburton said the current World Rugby rankings reflect the shifting power balance between north and south.
“After we had only southern-hemisphere semi-finalists in the last World Cup, the northern hemisphere teams have caught up. It is certainly the most competitive they have been going into a World Cup.”
Warburton added that Ireland had “more than justified being No 2” but New Zealand “still deserve to be No 1”.
The former loose forward believes that the aura of invincibility is wearing off from the men in black.
“I do not think that New Zealand played especially badly [during their loss against Ireland last weekend], and that can only be good news for the rest of the world. You can never write New Zealand off, but it looks like teams can now figure out a way against them.”
Warburton earlier this year announced his retirement at age 29 after 74 caps for Wales and two tours with the British and Irish Lions, whom he led both times.
Find out more about Japan, the host of next year’s World Cup, with our exclusive city guides:
Sign up to our mailing list here and we’ll keep you up to the minute with weekly updates from the world of rugby.