Turn on Mac with Tasker
Yes, you are talking about Wake-on-Lan. And as far as I know, the device to be awoken this way must be plugged-in via an ethernet cable (how else? when it's turned off, so is its WiFi).

As for the network: either both devices have to be in the same network, or you have port-forwarding enabled on the router connecting the different networks (otherwise your target cannot be reached).

Now for the remaining part: How to use your Android device to trigger this? You were asking for Tasker -- and yes, that should be possible. There is a Locale Wake On LAN Plugin (and Locale plugins do work with Tasker. I cannot see that Tasker natively supports NFC -- but there's e.g. a Youtube video, so there must be a way -- hm, maybe this Locale NFC Plugin?

So now there should be everything complete, and you can start testing :)