It's not so much about the absence
in the poems, it's more about the message of absence. (Minor shift, but consider absence as a theme/V&V and not just as a structure of the poems.) I'll only list a couple of things here to get you started, but the primary concern would be the absence of a generation (ie. the fact that so many went to war) which most of his work explores in some way. You also have the absence within a person; the absence of emotions and sanity seen in his later PTSD-centric poems. At a stretch you might even consider the absence of morality in sending young men to war, though this is more abstract.
If you can find evidence of loss, you should be able to find absence. After all, once you've lost something, you're left with an absence, right? So how does this come across in the poems; what is lost, exactly?
As I've said a couple of times, this is how VCAA will often try and trick you. By throwing out a word like 'absence' that you haven't seen, dealt with, or even considered before, they're hoping to separate the people who can only deal with a few themes from those who are capable of using what they've learned and reformulating it to suit a different discussion