If they only eat plants then it is a closed cycle. The only CO2 they breath out comes from the plants they eat, which breath in the CO2 to grow. In effect, they are a lawn mower that is fuelled from the grass they cut.

Remember if you cut the grass by any other method and just leave it, then it rots down to produce CO2 and methane through bacterial decay.

With the possible added bonus of wallaby burgers after!

and much cuter