Just recently went through this myself. First Medical, Class 3.
As I failed the Ishihara (never been able to do them, until someone tells me the answer then I can see them occasionally), got a letter from Transport giving me the alternatives.
You can do the Farnsworth D15 at an otometrist of your choice, or do a lantern test - If I remember correctly there is an office at Buttonville that can do it.
I chose the Farnsworth, as I knew I had previously passed it - went to my local optometrist and $10 later I had a letter to send to Transport. A few weeks later, an unrestricted Medical arrived in the post!
Try this - 2 good points.
1) It seemed to me very cheap
2) If you fail, you don't need to tell anyone!