Hi @Ez,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
You look to have a couple of issues going on. The yellowing and dying leaves, as well as some pests eating your leaves.
I suspect that the yellowing leaves are due to a nutrient deficiency, likely magnesium or iron as you have said. As you have said you are applying fertilisers aimed at Gardenias at regular intervals, I suspect the issue is with the plant's ability to intake the nutrients which could be due to the PH of the soil.
High soil pH (alkaline soil) can block nutrient absorption in Gardenias, especially iron and magnesium, leading to yellowing and browning. I'd suggest grabbing a Manutec Garden Care Products Soil PH Test Kit and checking your soil. Gardenias like slightly acidic soil, somewhere between 5 - 6.5.
If you need to lower the PH, apply Manutec 1.5kg Sulphur Fine Granules following the instructions on the package to increase the acidity. If you need to increase the PH, use Richgro 5kg Natural Dolomite Lime to increase the alkalinity.
Along with the soil PH, you should be wary of how well-draining your soil is. Waterlogged soil can cause the roots to become oxygen-starved, leading to yellow leaves with brown, crispy tips. Ensure the top 2-3cm of soil below the mulch is dry before watering.
Mix in compost such as Osmocote 25L Scotts Compost Premium Soil Improver mixed with Ki-Carma 20kg Coarse Landscape Sand to improve drainage and soil structure. Use a small garden fork such as this Cyclone Stainless Steel Fork Hand Tool to loosen compacted soil, being careful not to damage roots. This will allow better water flow and prevent water from pooling around the roots.
In terms of pests, have you noticed any small insects on the leaves?
If you spot any, I'd be happy to recommend a remedy, but you would need to identify what we're dealing with before I could recommend something.
Allow me to tag @Noelle to see if she has any further recommendations.
Jacob
