4 in 5 - pretty good odds. You'll be fine! And if you are really unlucky, by the time you need them there will be more effective preventative treatments and possibly even repairative (not to mention the possibilities of CCSVI).
At the moment you may be getting enough vitamin D from the Australian summer sun, although sunscreen is apparently very effective at preventing synthesis, if you happen to use it. I take 10,000iU in summer and 20 in winter. Oh and I forgot, it's D3 that's needed not D2.
A collection of articles:
http://www.msrc.co.uk/index.cfm/fuseact ... ageid/1334
But this excerpt, way down the page from 'Researchers explore the benefits of supersized doses of Vitamin D for Multiple Sclerosis and other diseases ' details the study which brought my attention to the need to replace the low supermarket dose with high doses:
Researchers are taking a fresh look at vitamin D - the over-the-counter supplement much-ballyhooed as a way to prevent diseases - to determine whether it could be effective as a medical treatment for those who already have chronic illnesses such as cancer.
Although the investigations are in their early stages, any successful outcomes could be a major health breakthrough, giving patients an inexpensive treatment option that's as close as the nearest pharmacy.
Already the so-called sunshine vitamin is glowing brightly in medical circles, with recent studies showing its efficacy in preventing everything from cancer to the flu. Typical of the recent investigations was one conducted at St. Michael's Hospital in Toronto, where multiple sclerosis patients received one of the largest vitamin D doses ever dispensed in a clinical setting.
It yielded some tantalizing evidence that supersizing the nutrient helps calm symptoms of the neurological disease.
The MS patients took up to 40,000 IU daily, or the amount in 50 multivitamins or 400 cups of fortified milk.
(The Canadian Cancer Society recommends taking 1,000 international units daily.)
"We definitely had fewer episodes in the treated group," said Jodie Burton, the principal investigator.
Because no one knows the dose most helpful for MS, Dr. Burton's group wanted to establish first that it was safe to take a lot of the vitamin. Although overdoses are rare, it is possible to take too much, with symptoms including excess calcium in the urine and blood, cardiac rhythm disturbances and kidney damage.
The researchers didn't find these problems. "We saw absolutely no evidence of any issues," Dr. Burton said. "What evidence there is suggests that you can go quite high with this before people start to report side effects."
Many scientists have been wondering whether a lack of vitamin D causes MS because the disease is far more common in countries, such as Canada, where people have low levels of the nutrient in fall and winter because of the seasonal drop in sunlight.
Although vitamin D is available in supplements and is found naturally in some foods, such as oily coldwater fish, most of what people have in their bodies they make themselves, through a chemical reaction that starts when cholesterol in skin is exposed to intense summertime light.
"There is a lot of evidence that suggests if your vitamin D status is really quite good, the risk of getting MS is low," Dr. Burton said. "The natural question after that is, 'Well, what happens after the fact, if you already have MS? Is there any benefit to vitamin D intervention?' "
The researchers gave the vitamin in escalating doses for a year, starting with amounts under 10,000 IU and gradually increasing intakes to peak at 40,000 IU a day. They then reversed the process and slowly lowered the doses to zero.
Averaged out, the patients received 14,000 IU a day, not far off the estimated 10,000 IU a day that people make in their skin if they live year-round in a sunny climate and spend time outdoors. A separate group of MS patients, known as a control group, was able to take up to 4,000 IU a day, the amount Dr. Burton says is the standard recommendation at her clinic.
Those in the trial took the vitamin as a concentrated liquid. The doses had about the same volume as a teaspoon, saving patients from swallowing hundreds of pills.
The amounts were far above Health Canada's recommendations of 200 IU to 600 IU a day, depending on age. The agency also pegs at 2,000 IU daily the safe upper intake by those not under medical supervision - although the MS research suggests the government's limit may be far too conservative.
The article then goes on to detail the effects of vitamin D in preventing various cancers (everyone ought to take it I think. The statistics of just how effective it is are impressive.)
I thought it might be more helpful to hear it from people first hand. 
