They may be undefeated but Jack Ziebell isn’t happy about some elements of North’s start to the season. The Roos are the league’s most potent scoring side, but have also leaked the sixth most points so far this season.

"Our offensive game is something we don't need to put too much work into, it's come pretty naturally for us over the last five years or so," Ziebell told ABC Grandstand after the game.

"But in the defensive game, if you're consistently allowing teams to kick 100 points, you're probably going to be losing more games than you win.

"It's something we need to address ... particularly this [coming] week against Gold Coast up there [at Carrara], it's going to be a big challenge for us."

Ziebell’s 26 disposals, seven marks, eight clearances, four tackles, seven inside 50s and three goals stands as one of his best career performances.

He said it was pleasing his side was able to execute in the heat of battle.

"There were plenty of skill errors out there but that just shows the pressure that was on," Ziebell said.

"The team that was able to stand up to the pressure was able to score. A few times we turned it over, a few times we forced them to turn it over - but in the end we're happy to take the four points."

While North’s 31-point win over Fremantle made another statement, but Ziebell says his side is yet to show its best.

"It's always nice to be 4-0," Ziebell said.

"We've had a pretty even start to the year - we haven't played our best footy in the first four rounds, but it's nice to get the job done and be sitting four-zip."