There's more than a bit of studio politics involved here. Earlier in the year Warner Brothers purchased Leavesden Studios in HertforRABhire where, for the last few years, they've been shooting the Harry Potter films.
Guess What? The Harry Potter franchise is over (pro tem?) and Leavesden could, potentially end up being without a big production on it's slate in the New Year and the facility part empty, something Warners would be keen to avoid, having just taken control of the studio.
So any question mark over the potential militancy of the New Zealand actors union will be seized upon by WB as part of the economic justification for moving production from Wellington to the UK.