Geelong coach Mark Thompson warned his team on Sunday it will have to play a lot better next week against fellow unbeaten AFL rivals St Kilda if it is to keep its unbeaten start to the 2009 season intact.

The Cats won their 13th successive game to start the season and their 55th in their past 58 matches by beating Port Adelaide by 34 points on Sunday despite being outscored in two quarters.

It was another unconvincing performance by the Cats, following recent close shaves against West Coast and Fremantle, but Thompson is hoping next week's clash against fellow league heavyweights St Kilda - in what is one of the most eagerly awaited AFL home and away matches in years - will spark the Cats back to their best form.

"The thing that concerned me today was how we fluctuated so much in one game of footy," Thompson said, after a day in which it was announced he had signed a new, two year deal to remain at Skilled Stadium until the end of 2011.

While the Cats set up their win with a seven goal to one first term and a six goal to one third quarter, Port actually kicked five goals to three in the second term and stormed home with six goals to two in the last although the home side was never seriously challenged.

But the inconsistency was enough to concern the normally unflappable Thompson.

"We can’t afford to do that next week," he said.

"But the challenge and excitement of playing the top team in the competition (St Kilda) will automatically get the boys up for a pretty solid performance."

Another disappointing aspect of Sunday's regulation win was that tall forwards Cameron Mooney, Tom Hawkins and the normally reliable Steve Johnson could only manage one goal each despite the midfield dominance of the Cats - led by another best afield effort from Gary Ablett.

But Thompson was unconcerned by that aspect of his team's display.

"We still scored (at least) 100 points pretty easily and as long as we are scoring goals and everyone is playing their roles…," Thompson said.

"We actually flirted today with getting Mooney and Hawkins out of there (the forward line) for a while and putting Gary (Ablett) deep (into attack) so putting other people down there (in the forward line) was a bit of a change and it worked well for us."

The Cats lost defender David Johnson with a strained calf on Sunday - an injury likely to sideline him for at least three matches - but number one ruckman Brad Ottens is only two weeks away from resuming from the knee injury he sustained in Round 2 against Richmond while Thompson said he was pleased with the performances of both of his back-up ruckmen in Shane Mumford and Mark Blake against Port on Sunday.