New turf, how long until you fertilise it?

Without knowing the region of the country the turf is planted in, it's hard to tell. Why? Because soil types and soil chemistry vary from place to place. Usually yellowing is not due to lack of fertilizer, but iron deficiency. There are soil test kits you can get from your garden center that will tell you.

I live in the Southwestern US where our largest water source is an aquifer that depends on water filtering down to recharge the aquifer. Since it's the high desert, I have only a small lawn about 12' by 15' I don't use chemical fertilizers at all since their residual chemical components would eventually leach into and contaminate the aquifer. Over the years I have used mushroom compost one year, alternating with deodorized steer manure the following year. They are cost effective with the biggest bang for the buck.

Also I'd recommend making use of your area's Master Gardeners volunteer groups or your local county extension office.

Thanks. I'm actually in Australia. There is a good organic fertiliser that I've seen, I may just go and buy it and give it to them.