MELBOURNE coach Paul Roos has questioned his side's effort and ability under pressure after a disastrous second half in the Demons' 45-point loss to Greater Western Sydney at StarTrack Oval in Canberra.

The Dees started brightly in the nation's capital when they kicked four unanswered goals in the opening term in a clash between two of the league's youngest lists. But Melbourne was embarrassed in a third term that saw them give up a 27-point lead, as the Giants booted nine goals in a canter.

Roos says there was a simple reason for the turnaround; his team just didn't have a crack.

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"It's an effort game isn't it? We had really good effort in the first half, then our effort capitulated in the second. Theirs picked up, and that's what footy is," Roos said.

"The most basic fundamental of footy is giving 100 per cent effort, and we're not a good enough team at the moment not to. We just don't have the talent to go through those patches where we're not giving effort.

"For us it's not a habit yet, to do it over and over and over again, and that’s part of the learning process and part of getting better as a footy team."

The scoreboard flattered the Demons at the first two breaks. The Giants led the inside 50 count 27-19 at half-time, but the visitors repelled many of their attacks with ease.

Melbourne's defence stood firm with Tom McDonald on top of Giants star Jeremy Cameron, and enjoying good support from Colin Garland and Lynden Dunn, while midfielder Jack Viney had put the clamps on GWS jet Ryan Griffen.

WATCH: Click here to watch Paul Roos' full press conference

Once the teams ran out after the main break however, the Demons looked a different side from the one that eclipsed the Suns last week.

"It's been a pattern here over the last couple of years. They kicked nine in a quarter and six for the rest of the game, so as soon as our effort level dropped to that standard, you get smashed," Roos said.

"We tried to put one back and slow the game down, but ultimately it comes down to us coaching and obviously the message isn't getting through, and we have to coach better than we are at the moment.

"You can throw some tricks in there and make some moves but at the end of the day the team that tries the hardest generally wins.

"If you don't have effort the game falls apart completely.

"The third quarter was shocking but credit to them, they lifted and we just capitulated."

Roos dismissed suggestions that the Giants were better prepared physically, but instead again reinforced his belief that the Demons' problems are above the shoulders.

"I think everyone can run a little bit harder than they think they can run, we're no more or less fit than the Giants. I don't think fitness is an issue," Roos said.

"I think it's just that mental side, how hard the game is, and how hard the game will continue to be."

While disappointed in his side's effort, the Melbourne coach paid credit to his opposition, and says the Giants will continue to cause problems for the competition in 2015 and beyond.

"I think we know the talent level, the way the competition's been set up with all their early draft picks, but you've still got to be able to coach them," Roos said.

"I thought the addition of Griffen and Patfull was really important. They've got quite a good balance of experience and young guys who are definitely developing and they're a dangerous team because of the talent level they have."