European rugby teams are still three years behind southern hemisphere sides and only England look like a threat at the World Cup in October, says New Zealand's top referee Paddy O'Brien.

O'Brien, who refereed the Ireland-England game in this year's Five Nations Championship, said European sides had adapted better to the stricter tackled-ball law interpretation and were playing a more expansive style.

"The game's flowing better in the northern hemisphere now but I still think they're three years behind," he said.

"Despite their loss to Wales I still believe England are the only side capable of competing with the All Blacks, the Springboks and the Wallabies because of the strength and size of their pack. The defences in this part of the world and in the Super 12 are so much stronger than what we've seen in the Five Nations."

O'Brien said Scotland, Ireland and Wales would not get as much room to run against New Zealand, South Africa and Australia.