I guess I have to wonder if the fertilizer is being applied regularly, and according to instructions for containers. Can you be specific about the fertilizer? Is the soil moist a half-finger down? The soil surface looks dry, so I suspect you aren't overwatering.

The fertiliser is 'Yates Thrive: Tomato & Vege Natural' I applied it as per instruction.

I've watered on dry days but we have had some rain which too. The soil is moist to half finger down at least.

Full sun approx 6 hours.

How would I determine if they are being over watered? I guess there is a possibility of the pots not draining properly

That's plenty of sun. If the soil surface is dry, they're not obviously being overwatered. But yes, if the pots are not draining properly, that could make for big problems down below. I guess the question becomes whether the soil turns to mud deep down. A good potting mix shouldn't pack up and hold water in, so you might ask yourself if the holes in the pots are clogged.

"The soil is moist to half finger down at least."

Then there is no need to water. If you stick a finger in and feel that it is moist then there is still plenty of water for the plants to take up. If you are watering even when you feel moisture then you are almost certainly overwatering. The dryness of the soil surface has nothing to do with how much moisture is actually in the soil. The surface can be dry while underneath is still plenty moist.

What are your temperatures down there in New Zealand?

Rodney

Currently between 10c and 18c

10c is a bit chilly for a cucumber.

The lettuce actually look OK to me. The centre leaves are crisp and they're the ones that show you if the plant has transplanted well. How long ago did you plant them?

I think the OP said lettuce and tomatoes. But yes, tomatoes will not thrive at those temperatures. They'll live fine, and grow, but they will do so slowly. The lettuce does look pretty yellowish. Did you check the pot drainage?

The OP did say tomato. But the second picture shows a sad little cucumber. It’s even labelled as such. That’s why I mentioned it.

Good point, but I see no labels. I don't see a lot these days, thanks to Houzz. They're looking into why.

The label’s at the side of the pot half out of the picture. I’m using an iPad and it gives eexcellent magnification. Better than my pc.

That's interesting. On my Mac with Firefox, I only see a piece of the picture. That label is in barely it, but almost illegible. If I click on the picture, I can see more of the picture, with better magnification. That's a pretty small label, though. Good eyesight!