Hi @SarahO,
A warm welcome to the Bunnings Workshop community and thank you for your question.
I'd suggest you have a quick read through How to diagnose a sick plant from @Noelle our resident horticultural guru, this will likely shed some light.
Start by inspecting the plant for fungus or insects. I can't see evidence of this from the photo, but either of these things can cause yellowing and browning of leaves. It's unlikely, but it's worth confirming there are no issues on that front.
The more likely cause of the yellowing is over or underwatering or a nutrient deficiency of some kind.
Pull back the mulch and inspect the soil around the base of the tree. Overwatering can certainly cause this issue, so if you find the soil is quite damp, I'd suggest reducing the amount of water you are giving it and keeping the mulch back from the base of the tree.
Nutrient deficiency is also a potential cause which can be addressed with an application of Scotts Osmocote Roses, Gardenias, Azaleas And Camellias Controlled Release Fertiliser. Ensure you follow the manufacturer's instructions about application rates.
Gardenias like slightly acidic soil, so checking the soil PH with a Manutec Garden Care Products Soil PH Test Kit can also help direct your approach. If you find the soil is not in the correct range between 5.0 - 6.5 we'll have to either increase or reduce the PH. You can reduce soil PH by adding acid in the form of Sulphur Fine Granules
and increase it by adding an alkaline in the form of Natural Dolomite Lime.
You might also like to have a read through Growing Gardenias: How To Grow Gardenias for some additional guidance.
Allow me to tag @mich1972 to see if she has any other advice to offer.
Let me know if you have any further questions.
Jacob
