Thinking it must be a spider bite we did a lot of research. After visiting the doctor in Rotorua it was obvious it was a boil.

HERE ARE THE PHOTOS

SPIDER BITE INFO:

The bad news is "There is no cure for necrotising arachnidism." as stated in a very good Victorian Government publication. here

The quote below reiterates this point and is from The Australian Venom Research Unit . Here is the full article.
Treatment of necrotising arachnidism
There is as yet no definitive treatment for necrotising arachnidism. Those that have been tried include antibiotics, corticosteroids, hyperbaric oxygen therapy, surgical debridement +/- skin grafting (early or late) and (for loxoscelism) cytotoxics such as colchicine and cyclophosphamide. Antibiotics that have been used include dapsone (particularly in the United States), erythromycin and doxycycline, as well as penicillin , flucloxacillin and cephalosporins. None of the above treatments has been systematically trialled. Alternative treatments such as L-cysteine and aloe vera have also been used. There are anecdotal reports of success in healing ulcers, including ulcers thought to be related to spider bites, with hyperbaric oxygen therapy, but no organised trials have been conducted.

There is a spider bite support group here and the photos here so how bad it can become

Here is a very useful article on Necrotic Arachnidism from the Australian Wound Management Association from 2011

Here is a article from The Australian Medical Journal about Acute and recurrent skin ulceration after spider bite which indicates the variety of approaches taken and often rather ineffective.

Also other possible causes of similar lesions include Sporotrichosis (a fungal infection) and here is an article about that which also references a WA case of a spider bite being misdiagnosed.

Here is another interesting article specifically about white tailed spiders. The second short video are quite interesting.