This help?

Here is a link that might be useful:

Interveinal chlorosis (yellowing leaves with green veins) of new growth in tomatoes is supposed to indicate a shortage of iron.

It might not be a lack of iron in the soil, but something else interfering with the plant's uptake of iron.... Too much potassium, zinc, or copper can do that. High pH can also be a problem.

I'm not clear on the timing of the yellowing and of the chemical fertilizer. Had the yellowing begun before your holiday, or did you only notice it on your return? Was the fertilizer applied before you noticed the problem or after? If after, did it have any effect on the yellowing? And what's in that fertilizer?

[This is about as far as I can go. Someone else will have to tell you what to do about it.]

hi bro, if your soil ph is too high, many of the micronutrients (including iron) will be unavailable to the plants irrespective of how much is actually in the soil.

u should buy a ph soil testing kit, can get them at Bunnings for about $20. if soil ph is too high (alkaline) you'll need to add some elemental sulpur.

good luck!

Awesome advice, thanks everyone. I suspected ph but from my reading applying ample compost helps neutralise the soil? I do water with bore water that I know is quite high ph.
Off to town today to purchase soil pH tester. Reckon it will still be too late to help these plants now ey?

Cheers

Dan

Test has confirmed, water I have been using is ph 7.8.
And soil test is off the chart, around 8.2.
Bit gutting but at least I have found the problem.

Cheers
Dan