Are those the oldest canes, Mike? Bamboo canes rarely last more than 3 or 4 years. New ones should come up to replace them every spring. Also, what are you feeding them? Bamboos in pots are hungry beasts--as well as thirsty--and starvation will accelerate cane aging, as well as reduce cane replacement.

Thanks for the reply :)
They've only been planted for about 12 months - there's no pattern with older or newer canes becoming discoloured - it's effecting some in the middle of the clump and some on the outside.
I haven't actually fed them anything! (I'm a fairly new gardener here... managed to combat caterpillars, slugs, silver leaf miner and mites on my citrus trees in the last 12 months!)
What do I need to do to care for the bamboo properly?
The third pic I posted shows a very black section with a white spot - is this some kind of fungus or virus?

Well, here in the American Southwest, we water bamboo in containers 1-3 times a day, depending on the weather. As you can imagine, we also feed frequently, too. I have found foods with 3-1-2 proportions to be most effective. Liquid foods need to be applied every two weeks, or weekly in hot weather. Timed release foods are used according to package directions. I have found that bamboos need nutrients year-round: in early spring to support new cane growth, late spring and summer to support foliage and rhizome development, and fall through winter to synthesize proteins for the spring growth burst. We rarely have problems with diseases, due to the low humidity, but occasionally get pests, such as mealybugs or aphids, that are normally rapidly controlled by their natural predators. The latter occurrence depends on no preventative spraying with chemical pesticides, for the last 2-4 years.

That's great advice, thank you. I'll pick up some food this weekend. We're heading into autumn (fall) here in Australia, but have had a very hot and long summer - the mornings are cooler but it's still getting up to 27c here in Melbourne during the day which is unseasonably warm.