Tea tree oil, along with witch hazel has really helped clear up a nasty breakout I've had for the last couple months. I had fresh bumpy zits all over my cheeks and along my jawline, even some on my neck, and I'm finally getting relief. My face is smoothing out and the zits are drying out more and more everyday. It's really amazing to see my face finally clear up.
I mix up a toner containing 10% tea tree oil, 90% witch hazel, and apply this all over my face and neck with a hypoallergenic cotton pad, twice daily after cleansing (gentle BHA cleanser). Proper dilution is key! I use a graduated cylinder to make this exact dilution and keep the mixture in a small 3 oz. squirt bottle.
This 10% dilution is perfect for me and doesn't overly dry but is still effective against acne. I want to give my skin a chance to breathe while it's healing from this breakout so I moisturize just very lightly afterward.
Bottom line, tea tree oil is incredible and very effective at treating skin infections such as acne. You just need to be very careful though and find out what dilution works for you. I wouldn't advise using 100% since it could cause more problems in the long run due to dryness and skin irritation.
Use a dilution between 4-15% and you should be good to go! Anything less than 4% can be ineffective, and more than 15% can be overly drying. Try it, it really works!