ST KILDA played as well as it has for some time in the first half against North Melbourne, but its second-half fadeout, particularly in the third quarter, was unacceptable, according to coach Alan Richardson.

After conceding the opening two goals in Saturday's game at Blundstone Arena, the Saints piled on six of the next eight goals to take a 20-point lead into half-time.

Click here to watch Alan Richardson's full post-match press conference

Their lead should have been greater too, but they kicked an inaccurate 6.11 for the half, including 3.8 in the second term when they had 22 inside 50s to the Roos' five.

But after the two teams kicked two goals apiece in the opening 11 minutes of the third term, North turned the game on its head, kicking seven unanswered goals to go into three-quarter time with a 25-point lead.

A five-goal-to-three final term completed a 37-point win for the Roos. 

Richardson was buoyed by the way his emerging young team had been able to dominate a finals contender like North in the first half, but said his players had to learn to maintain that sort of effort and intensity for four quarters. 

"It's probably as good a footy as we've played in the first half for a while. We were really strong and really aggressive with our pressure," Richardson said. 

"We just weren't able to maximise opportunity - we got shots and weren't able to convert.

"I was really disappointed with the third quarter, that was really poor. We got beaten badly in contests and got smashed on the outside with spread. 

"Why was that happening? Why were we so strong and so aggressive and so up for the fight in the first half and then after half-time pretty easily pushed aside? 

"They're a good team (North Melbourne) - I want to take nothing away from them, but that doesn't stop us from applying that same energy and effort.

"That was our conversation after the game. There were not a lot of answers but there was certainly some conversation around the fact that if you're going to play and be a good team you've got to be stronger for longer."

Richardson was encouraged by the performances of first-year key defender Hugh Goddard on North spearhead Drew Petrie, Jimmy Webster on Roos goalsneak Lindsay Thomas and Tom Hickey's game in attack. 

Goddard battled hard in one-on-one contests with Petrie and although he ultimately lowered his colours to the Roos vice-captain, who kicked a game-high four goals, Richardson said the 19-year-old would get "a lot out of that game".

Webster kept Thomas goalless in the first half before the small forward got off the leash with three goals in the decisive third term, while Hickey was a constant threat in attack, taking seven marks (a team-high six contested) and kicking 2.2.

But Richardson said all three players had lapses at times in the second half and had to learn from them.

"AFL footy is a tough caper, you've got to apply really strong pressure and be good at your role for four quarters, not for a half," Richardson said. 

"It's not OK to take a really good mark and then turn around and kick it back to the opposition, or as a forward get the opportunity to score and not finish your work. 

"That's part of what your role is, you need to get that done. 

"We can't accept what happened in the third quarter, irrespective of how many games they've played, they need to learn from that and grow from that."