I live in Dunedin and have quite a large mixed border with approximately 30 roses. I'm finding it hard to suppress the weeds around the roses. Do you have any ground covers that you could recommend under roses? Or any other ideas?

Instead of groundcover I would recommend using peastraw. Peastraw is great for roses because they feed off the surface and the peastraw produces nitrogen. Simply use a hoe to dig up the existing weeds and then spread the peastraw onto the soil. Reapply it the following year. It will keep the moisture in during summer and provide fertiliser the rest of the year. Or you could try growing Catmint (Nepeta) among your roses.