Why do my Tomato plants turn yellow and die?

Hi Peter, Help please.  My tomatoes (leaves are turning yellow), does that mean they need more food, (what food) more water or cooler weather.  Yes I know our weather here in Melbourne has been dreadful, lost most of my garden, its nearly bad enough to make me throw up my hands and say DAME THE GARDEN, WHY BOTHER.   This is 2 years in a row for me, last year I nearly lost all my garden from the hail, (live in Greensborough where a lot of damage was done). My garden is still recovering from that, and now the horrible hot days, Anyhow, what about the tomatoes please.
Thanks Judi

I have dreaded this question because I see this happen so often at home.  Nothing I can do about hail, but we see plants grow beautifully until they are ready to set fruit then the damn thing turns yellow and dies. Sorry I don't have a pic today, but I bet you know what I mean.

There are all sorts of dire diseases that can attack Tomato plants, but I'll ignore them for now although it is possible that what I'm about to describe creates the opportunity for a given disease to infect the plants. I think the primary issue is plant stress.  This problem invariably occurs at the height of summer when we get extremes of heat, dryness then sudden wetness. I also think that a lack of fertilizer comes into play after the initial burst of growth.  All the fertilizer available initially is gone and the plant now needs more food as it concentrates on producing fruit, this is particularly the case with plants in pots.  We never imagine just how big the plant will grow when it is first planted and we use pots that are too small, most Tomatoes need a pot with at least a 400mm diameter. Also be prepared to add fertilizer, potting mix manufacturers under fertilize even the best mixes for fear that too much nutrient available in the early stages of growth will cause burning and growth problems.

One tip that I like is to prune off the foliage, starting at the bottom of the plant leaving only 3-4 leaves per stem.  This has a number of advantages including reducing transpiration, improving air flow and removing pest habitats.  You'll be surprised at how few leaves are needed to support Tomato plants once they are established and developing fruit.