Thank you for getting back to me. The injected penicillin treatment has been found to be the most efective against head and face abscesses in rabbits. However, if it is used for the usual 10 to 14 day treatment, that isn't long enough. It must be continued for several weeks.
It is not recommended to use tea tree oil in rabbits. It has been found to be toxic to some species, including cats. When it was tested on rabbits, it caused skin problems, and that's the last thing you need with an abscess. Used in higher concentrations, it can damage the liver. Some rabbit owners have used it anyway, in a very dilute form - 2 to 3 drops of tea tree oil in a tablespoon of carrier oil. The bacteria in abscesses are among the most difficult to destroy, and a weak tea tree oil solution is very unlikely to do any good. Because of this and the potential for problems, I cannot recommend it.
If the penicillin shots weren't administered for several weeks, you may want to contact your vet to see about continuing them. If they were given for a long period of time, the next step is usually surgery, followed by the implantation of antibiotic beads. You may want to discuss that with your vet. You can read more about treatments here:
It's a good idea for any rabbit on antibiotics to be given some beneficial bacteria (probiotics) to prevent the digestive problems associated with antibiotic use. A brand called Bene-Bac is often available in pet stores and online. You can also use a human liquid formula, if it is nondairy. These are available in health food stores. Give some beneficial bacteria every day while the rabbit is on antibiotics and for a week afterward. After that, provide some twice a week.
Only you can make the decision about what step to take next. If you have more questions, just let me know by clicking on REPLY. I hope you'll find an effective treatment for your rabbit.