In fact, now that I think of it I can email some friends of mine who live in the area and mountainbike (one lives in Porirua, another in Plimmerton and the other in Paraparaumu). I know they have biked in the area behind Whitby as one of them dislocated his shoulder there a few years ago and had to get helicoptered out.
I'll let you know if I managed to find out anything.
There’s a MB park at Otaihanga (Paraparam) near the tip. It’s small but lots of jumps and ramps. There’s also one I haven’t been to around Makara/Karori somewhere which is supposed to be great. There are lots of tracks up round the windmill at Brooklyn. There’s also the Mungatuks.
There are also heaps of trails in the hills between Hutt Valley and Porirua – Horokiwi / Korokoro / Maungaraki / Belmont regional park.
And don’t forget the Rimutaka incline track which goes from Kaitoke over to the Wairarapa.
I think you may have to discuss the above with a local to find out where all those places are. Have you tried asking at your LBS?