How to remove a mildew stain from a shirt?

If that shirt is really that expensive because it came from Paris. You should go to a professional cleaning service that have top quality service. There is only one shop that I know off, check this:

http://lawrencedrycleaners.com.au/ They provide quality service in different types of clothing, linen, etc. Try to look if there is a shop near you. Or go to a same shop that has the same quality service.

If you're still looking for do it yourself remedy, here it is:

First bring the clothing outside to prevent the mildew in spreading inside your home. Treat it with a heavy duty liquid detergent ( compatible with colors). brush item with this solution and do it at least for 15 minutes to make the solution sink in the fabric. Yes you still need to use bleach but used Oxygen bleach or any type that is safe to use on colored fabrics to remove the stains. repeat the process until it is removed and dry clean.

If you are still have doubts in doing it, I advice you to take the first option. Its up to you

Good luck!

Tea tree or clove oil will kill the mould. Try soaking in a colour safe oxygen stain remover solution for a while (they usually come in big pink plastic pots) then wash it with tea tree or clove oil in the wash. You might need to wash in a biological washing liquid too. Keep it all on a cool wash so you don't set the stain.