With only 2 watering days a week in Perth (3 days if you have a bore) and sandy soils most of which have not been improved or not improved well before laying a lawn then it is not surprising that we have issues with water penetration and retention especially as the rain stops and the weather heats up over summer. 

Lack of water getting to the root zone is the first place to start. Make sure that your retic system has even head-to-head coverage and is running long enough to deliver 10mm of water evenly over the entire lawn during it is watering session. This can be done by placing catch cups over the lawn or tuna tins work just as well and measuring the volume of water caught in these during the stations run time. If this is uneven fix your sprinkler positions and/or types of Nozzles (MP rotators can compensate for bad retic design in some cases) or you may be required to add new sprinkler locations and move existing ones to get good even coverage. If watering is even but not enough then just got to the controller and increase the run time as nozzles like MP rotators can run for up to 1 hour per station to deliver the 10mm you are allowed. If your pressure is low, then even with standard nozzles the run time might need to be increased to 12-15mins to deliver 10mm of water. 

OK so the watering is not an issue so we will now look at the soil. When was the last time a wetting agent was used? A wetting agent is a surfactant that is used to break the waxy coatings that build up and coat soil particles. This is a significant issue in WA especially if no clay was added to the lawn prior to laying. During the height of summer, I like to do monthly applications of Eco wet wetting agent but Aquaforce or some of the other products on the market are good. I do not use granular because all the turf professionals I have followed or spoken to use liquids as they are easier to apply and more effective. 

There are other products called water retaining agents which are polymers that coat the soil particles and roots that help harvest or attract water to them that can be added after you have applied a wetting agent and it is best with some of them to wait a few days after applying the wetting agent for products such as Biagra just to make sure that the wetting agent has been had enough water applied(15mm) to get it to the root zone. 

So now we have eliminated water being the cause of your lawn going dry and crispy make sure the lawn gets a good water with the retic, followed by hand watering the next day and wait a couple of days to see how it responds. 

If this does not fix the issue, then the next step is to look at fertilising/nutrition and if that is not the issue then look at pests and diseases but if the issue has come up as the weather has increased in temperature then water is most likely the source of your problems.