I'm in Sydney, but I have friends who garden in FNQLD so we talk about this often. When we have little seedlings - either freshly planted in the garden bed or still in pots - I let them get the morning sun only, then I cover them with an umbrella. Lettuce is hard to grow in QLD, so it might take you a season or so of trial and error before you get it right.

Lots and lots of water is a good thing in QLD. Because I have serious issues about wasting water we use grey water from our washing machine, and find that to be quite adequate. Watering in the morning - but only on the soil, not on the leaves - and then again in the late afternoon, early evening, will also help keep things going.

Have you mulched? Mulch will help keep root systems wet and insulated from the heat. Newspaper and mulch will get rid of weeds, and help the water retention too.

Failing that a shade cloth that allows morning sun, but not afternoon HEAT sun is what they need. The morning sun stimulates the plant to grow, the afternoon sun just melts them, even in Sydney.