Pre manufacturing of fully galvanised balustrade sections is common on higher end houses, but unfortunately more budget orientated houses are often produced cheaper. This results in the issues such as in the picture above. Total replacement is required, costing many thousands of dollars.
Each balustrade section requires measuring up, manufacturing, galvanising, powder coating then fitting. Most balconies have several pieces, so it's an expensive project.
If your'e looking at buying a coastal home and you notice a bit of rust, think of the consequences and expenses that may be involved. Or better yet, pay for an inspection from some one who knows what to look out for, so then you will know what you are really buying.