Robyn, I've had migraines all my life. I've just come upon sulfite. There are a few good web-sites about it. I's an intolerance rather than an allergy and for for some of us avoiding it in more than small amounts prevents the triggering of migraines. I don't know about the rules in NZ but here in Aus. it's used widely. The rules are stricter in the USA (making imported icecream OK for me). It's listed here either under it's own name -- and it's a naturally occuring preservative so "natural" is not necessarily helpful on labels -- or as "preservative number 202".
It's in all margarine-type spreads (except one), fruit juice, cordials, dried fruit, fresh and dried bananas, grapes, some cereals, and in all glucose syrup (used in all icecreams, some jams, honeys and toppings, and many sweets). It's also in lots of processed food. It's a presevative in some medications too.
Hope this is helpful to others. It's been a happy revelation to me.
Cheers, Joy