NikT: I've heard an anecdote (I think from the TradeMe forums) that someone with this issue was able to go directly to Samsung in Auckland & get a replacement battery on the spot. Hopefully with this clearly being a known fault (Colleague had the same issue, so did Murphy), the process is streamlined considerably. Good argument for user-replaceable batteries, I suppose.
The following was in the Vodafone Community Forum, posted on 30/12.
The battery had a bit of swelling over Xmas period and earlier this morning called up Samsung Service Centre up in North Shore and explained the issue to the rep. The rep said to come over and swap out the batteries. Turns out to be the batteries which were made in china were defective. The whole process took about 15 mins and so far there hasn't been a boot loop with the newer battery. http://community.vodafone.co.nz/t5/Android/Samsung-Galaxy-S4-4-3-Update/td-p/109748/page/6