Hey Joeyvek,
Carpet Grass forms into 2 main species: Broadleaf Carpet Grass (Axonopus compressus) and Narrow Leaf Carpet Grass (Axonopus fissifolius). People often get confused and call it Buffalo (Stenotaphrum secundatum) (Australian Buffalo) when it isn't and performs quite different in its response to various stresses such as drought etc. In Brisbane you will most likely find Carpetgrass growing in the shade or in a wet/boggy area of your lawn and sometimes quite acidic soil (low ph) (I have seen it growing around the 3-4 ph mark in North QLD and SE asia). If you go up to North QLD it is everywhere. Narrow Leaf carpet grass grows on most roadside verges and invades a lot of community sportsfields up in Brisbane and often gets confused with the weedy paspalums (notatum and dilatatum).

This is broadleaf carpet grass
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This is narrowleaf carpet grass
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If you don't mind it that's cool (I have seen it reel mown on fairways in SE asia its called 'Cow Grass') and it looks sweet. If you want to spray it out and you have green couch use DSMA or MSMA but if you have a blue couch lawn these will kill it. For Blue Couch use Bromoxynil (this may or may not work I have read it in a paper before). The other options are hand weeding or non-selective herbicide (round up) and growing in what you desire. Remember though most lawns are not a monoculture from the environment and cultural practices they are placed under. Weeds can often paint a picture as to what may be happening underneath in your profile. I.e your soil may be compact, pooling water on the surface making it conducive to the Carpet Grass Growth.

Cheers