Gardenia buds browning and dropping off from apparently hralthy plant.

You may check the following aspects which may cause the dieback of the tree: 

- Soil drainage, plants turn brown after heavy rain due to lack of air in the soil. Best to improve soil structure by digging in garden compost and Dynamic lifter into the soil. 

- Pest such as bud worms bore into the flower buds. To control, spray the plant with 'Yates Success Ultra - Insect Control', this product contains spinetoram, the latest generation insecticide derived from beneficial soil bacteria. 

One of the many benefits of this product is, once sprayed on to the leaf and dried, it is absorbed, helping the product to become 'rainfast'. 

Click here for more info on this product:

https://www.yates.com.au/product/garden-pest-control/yates-success-ultra