hm... personally I wouldn't want to live in Porirua itself. The shopping centre side is ok (but that sall thats there - the mega centre and a mall), but the other side is, well, a bit 'rough' to put it bluntly.
When we were in Welly I knew of a few people who parked their cars at the train station to commute to work and they got broken into etc. I certainly wouldn't have wanted to live there. So just be careful when looking at renting/house prices etc. They're a lot lower for a reason!
That said, Whitby, Paremata and maybe even Plimmerton are classed as Porirua I think but they're away from the rougher side and they're all very nice and near the water (but the prices are a lot higher to show for it).