Thanks for the answers to my questions and the pictures.
I suppose this could be a spider bite but it doesn't look like I'd expect if that were the case although you may have some nasty critters in New Zealand that I don't typically see in the States. It's also possible that he stepped on something and that he has a small abscess or infection which is present....or even inflammation.
Regardless, the following is what I'd suggest that you do:
1. Soak the foot for about 10 minutes or so in a warm Epsom salt bath several times a day.
2. Try to prevent him from licking at it if he starts since he may just make it worse. He may need to wear a cone although I know he won't thank me for the suggestion.
3. As long as he's not vomiting nor currently taking any steroid or non-steroidal anti-inflammatory medication, Aspirin can be given which can help with inflammation and pain. The dose would be 10 mgs per pound of body weight twice a day, with food to avoid stomach upset.
4. On the off chance that this might be a bite or sting of some sort, then Benadryl (Diphenhydramine) can be given although sedation may be seen as a side effect. The dose would be 1 mg per pound of body weight twice a day.
If the area continues to be swollen and/or bothers him, then a vet visit may be prudent.
I hope this helps. Deb