I've been searching for this option for a while but I'm not sure if I can do this.

I'm planning on using WHMCS with WHM and billing any bandwidth overage manually since WHMCS can only bill per MB and I want to bill per rounded GB.

However I can't find a way to be notified when a certain client exceeds their bandwidth limit. Not in WHMCS nor in WHM. I know a mail is sent to the contact e-mail by WHM, but then only the client receives the notification.

Admins need to be notified whenever a client goes over their bandwidth quota. How would I do this? Or better yet, how would I go about automatically billing overages in WHMCS per GB instead of MB?

You can choose to receive "admin" emails for users approaching or exceeding their bandwidth usage.
Go to WHM/Server Configuration/Tweek Settings/Notifications

I was under the impression that this setting controlled when clients receive a notification, not the admin?

Usually, large web hosts disabled such kind of tweak setting for them that notify admin which client's bandwidth limit is going to be exceeded, because if they enable it, then just imagine how many emails admin will receive.....it would be simply annoying....It is a responsibility of clients to contact web host............

Okay that makes sense, but that puts me in a situation where I dunno how to bill for overages. Should I even bill for overages of bandwidth or just make the client responsible to monitor it themselves?

I would say you charge for overages. In WHM, you can easily list clients that are over their BW limit. Do this on the 29th of each month and bill your clients exceeding those limits.

Ah that's a great idea. Thanks! I just checked out the WHM demo and found that option. Great!

In the WHM Tweak Setting I believe it is you can setup when notices are sent and to whom.