Hi @Llypilly,
A warm welcome to the Bunnings Workshop community, it is fantastic to have you with us.
The browning leaves look suspiciously like sunburn to me. Sunburn will happen when your lilly pillies are exposed to a lot of sun and don't have enough water.
While watering once a week may be enough during the intermediate and cooler months, during particularly hot stretches, you may need to increase the amount of watering.
Do you check your soil for moisture before watering?
If not, I would suggest you check the soil by putting your finger around 5-10cm into the soil at the base of the tree. In the hotter periods, you will likely find the soil dries out much faster than it does in the cooler months.
For a week, I would suggest you check the soil daily to find out how long it is taking for your soil to dry out and adjust your watering schedule accordingly.
I would also consider adding a plant tonic such as Seasol Complete Garden Health Treatment while watering to give your lilly pillies a boost while they are struggling a little bit.
Allow me to tag @mich1972 and @Noelle for their thoughts.
Jacob
