jm3: Maybe a ground barrier is necessary if you have kikuyu, we had some californian thistle pop up in a raised bed once. Generally not need though.
Instead of rocks to fill the volume you could use old tree branches/logs even firewood, they are a good moisture trap. Google 'hugelkultur bed' which is sort of the theory behind it.
Thanks very much. We are in Kapiti beach area and have kikuyu, plus the raised beds are going on top of old level vege beds that have a history of weed issues, so keen to separate the new beds from the old. We have a lot of logs, so will look at hugelkultur.
Cheap polythene weed mat will probable get clogged up completely over time which is not ideal. You could use a geotextile so water can pass through more easily, You can also get jute/wool/felt mats that will suppress weeds probably long enough to get on top of them and then they will biodegrade. If weeds are ingressing from your lawn you may need a root barrier at the edge.
It really depends how much money you want to throw at the garden, it's easy to get carried away.
I would place your matting whatever type you choose and then put a decent layer of pumice or scoria on top of it. The is will allow drainage, stop the mat clogging and the volcanic rock holds moisture, now start building your soil layers.
If your previous garden bed really is a problem and you want a closed system you could try a 'wicking bed' which I have been meaning to do for a while.