Please help save my navel orange tree!

Hi all, I’m a new citrus owner based in Perth. I bought this navel orange tree from a small gardening business who said the tree is about 5 years old. When I bought it, in June this year, it had one navel orange on it and looked really healthy.

Now it looks like this… certainly less than healthy. It has been dropping its leaves at a very quick rate since roughly end of August. These past two weeks, its lower branches have started turning brown/dying, which I have been trimming back. Its leaves are still green, no yellow on any of them, but they’re slightly curled up (but they were like that when I bought it).

think I’ve over-fertilised it, but here’s how I’ve been looking after it since being planted:
– A deep water once a week since purchase.
– Since beginning of August I’ve been fertilising it once a week with Yates Thrive Citrus liquid food.
– Beginning of September, I used Yates pellets Dynamic Lifter.
-Sprayed its leaves with white oil twice since buying.

It was planted by a landscaper whilst I was at work so I don’t know what soil he used. My garden’s soil quality is very poor. It was also planted in a spot in my garden with full, direct sunlight all day.

Please help! I’m a novice and I find the information out there Beth overwhelming.

(On mobile, sorry for formatting).