Just how bad is Air New Zealand's long haul aircraft shortage?

Air New Zealand have had a torrid time of late. With rumours abound of them returning to London, CEO Greg Foran has simultaneously confirmed their interest while pouring cold water on the prospect of them doing so anytime soon. They barely have sufficient long haul aircraft to fulfil their contemporary schedule, nevermind starting new routes!

Just this week NZ announced the cancellation of seasonal flights to Seoul citing “ongoing engine availability challenges across its fleet”. So what’s going on with NZ? Why are NZ struggling to maintain their schedule and why do they have such an acute fleet shortage?