After a bit of patient searching (thanks to NIWA, Wgtn Marine Education Centre et al for your help) I managed to find a large fish tank for sale on TradeMe. $500 instead of the thousands I was quoted to get one made.
Basically one metre cubed it literally weighs one tonne when filled. So after a massive cleaning operation and the construction of a very heavy duty base for it, the first nervous fill happened outside – whew – no leaks!
Massey have found me a storeroom with a concrete floor, no windows and a sink which I can use as a studio for a while. The glass is 15mm thick and very green which looks cool but may make it harder to give the illusion that the subject is not underwater – next step is large scale lighting tests.