Old 'Un: Having flown into and out of WLG in my younger years and knowing the area quite well, any extension would have to be into Evans Bay, purely from a cost basis. To extend at the other end would be prohibitively expensive, to say nothing of the issues of stabilisation, and salt spray in a half-decent southerly. It could be done without too much technical difficulty, I would imagine.
There in lies the problem. Extending to the northwest into Evans Bay isn't much help. It's the single engine climb performance to toward the north west that is the limiting factor now.
Extending the runway toward the high terrain effectively increases a climb gradient that is already a limitation. Or you just take off at a lower weight and get airborne at the same point as you do now, well before the new extended threshold and not use the extension at all.
To be useful the extension needs to be to the south east into Lyall Bay.