Hmmm...not sure of an answer for this one, sorry! It's not a situation I've come across before and is not covered in immigration instructions. I do know that a few temp applications that had been hanging around for a long time (for no other reason than INZ being overwhelmed with workload volumes + processing inefficiencies) were rushed through the process late last year (?) because the client's 6 month interim visas were about to come to an end through no fault of the clients.
I don't believe there's any allowance in instructions for a second interim visa to be granted but I think it would be worthwhile proactively asking INZ about this, just to be sure you don't fall through the cracks and become unlawful. The situation to become lawful again is much harder than getting things sorted before a visa expires (even if it's through no fault of your own!).