Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thank you for sharing your question about stoping water from entering your garage.
I can see where you have placed the drain and it looks like a good spot for water to drain away. If water is entering your garage have you thought about placing a drain channel in front of the garage to go into your stormwater pit? Putting a drain in front of your garage is an easier solution than trying to add more cement inside your garage.
Would it be possible for you to post a picture of the entire entrance of your garage? Is your driveway sloped causing water to flow down your driveway and into your garage? Is there no possibility of fixing the water table at the front of the house? Any other extra information you can provide will help us to make recommendations on how to prevent water from entering the garage.
If you have any other questions we can help with, please let us know.