Hey Genevieve, how large/wide do you require? Blundstone's largest size in gumboots is a 13UK/14US.
The largest safety workboots I know of available from NZ distributors is size 16UK/17US, although some brands offer a wider fit than others. Gumboots are available in 15UK/16US.
Happy to offer some suggestions if you PM me, probably against forum guidelines for me to list them here...?

Hey Michael, definitely be worth contacting Kelvin at Safesmart Access. They have some pretty innovative solutions for this kind of thing - 0800 000 448