If you think these cousins are in Australia you could try the phone book and either write or phone any Stapleton's. There are certainly Stapleton's living in Kilkenny, Ireland about 20 of them. No Mildwater's in Kilkenny or surrounding counties.
Again you can write as the addresses given in the phone book are sufficient for mail delivery.
You might be fortunate to someone interested at least. I understand Facebook is good for
finding those cousins. I don't know any more about Facebook than that. Ask a family member or friends about this.
Other than that, there isn't anything I can do for you. Good luck in your search. You might want to post on a Auzzie message board as well.