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. The floor of Evans Bay is rock, rather than sand or mud. If the extension was built as a pier deck on piles, the artificial reef created would improve the fishing there immensely.
Mind you, if Mother Nature has her way, we may not have to wait long before She does the work for us, given the seismic activity over the weekend. Remember the land upon which the Hutt Motorway is built used to be under the harbour's waters.
Le Vieux