Couple of things come to mind. Citrus are often quite susceptible to "transplant shock" and react by taking drastic action that the plant looks upon as self-preservation.
Watering well a few days before potting or planting allows the plant to absorb as much moisture as it can so it's well hydrated when you work on it. Citrus also dislike having their roots disturbed - they have relatively shallow, fibrous root systems and if these are damaged or unnecessarily teased out when repotting or planting, they will resent it and react by wilting, dropping leaves, etc.
When you pot it up, the process is to water it in thoroughly, then leave it alone for several days to settle into its new situation. Sometimes leaf drop may result from over-watering - that is, the potting mix or soil being kept too moist over a period of time. Roots need oxygen, so it is best to thoroughly wet the potting mix so there is water running out the drainage holes in the base of the pot and then not water again until the top 5 - 10cm of soil is quite dry.
If you’re heading into winter months, don't over-water and don't add any more fertiliser other than what was used when you potted the tree up. You could use a weak seaweed solution (seaweed only, with no added fertiliser or trace elements) to assist the roots to establish in the pot. The tree can survive over winter without foliage. If it doesn't produce new growth come September, then it's likely it will not survive. - Noelle
All our plants at Bunnings are guaranteed for 12 months, so if you’re not 100% happy, return your plant (with receipt) and we’ll refund it. This does exclude seedlings, as well as potted colour (bloomers). Fantastic bit of peace of mind and assurance when making a plant purchase. - MitchellMc