I have a weed in my lawn which is prolific where the grass is of low height. It is a very small clover with yellow flowers.
I live in the Sydney, Australia region.
Here's a photo with a ruler to scale:
1 inch = 2.54 centimeters
I believe what you have there is Yellow Woodsorrel (Oxalis stricta).
If it isn't, I'm 90 to 95% certain it's in the Oxalis genus, which Yellow Woodsorrel belongs to...