You can use Halosulfuron-methyl.
It is the active ingredient in Amgrow Sedgehammer, Sempra, and Apparent Halosulfron.
It is a selective herbicide for nutgrass and Mullumbomby couch, but works just as well on onion weed, and shouldn't bother the other grasses or plants (but always be careful anyways, with both to overspray and personal safety).
It is highly concentrated and also expensive. I picked up a 100gm container of the Apparent brand from www.catch.com.au for a very good price. For spot spraying, you only use 1.3grams in 10 litres of water per 100sq meters. Online is generally cheaper then what you can buy from Bunnings.
It takes take some time for you to see the results, but it does work. Also too, it works better with a non-ionic wetting agent too.
You may need to get some digital scales that can do very light weighing. Just beware if a cop pulls you over when you have the scales in your vehicle with a small trace of white powder on them, you may need to be doing some explaining.