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Can I please firstly direct you to the excellent D.I.Y. Advice article on the Bunnings website - How to grow and care for a lemon tree
I also suggest you share a photo of your tree and please describe it's conditions. Is it in a pot or in the ground? Does it get regular water? And when was the last time it got a feed? Any other changes the plant has experienced recently would really help our members to identify the problem and suggest a specific remedy.
Yellowing leaves generally mean a nutrient deficiency.
Depending on the type of yellowing it could be a nitrogen or other trace element deficiency such as magnesium or zinc.
I would feed the tree with a good citrus fertiliser that includes trace elements. There are several options at Bunnings.
Make sure you water in any fertiliser well and give the tree another feed every couple of months.
I keep a compost bin next to my lemon tree and I never empty it. I just keep adding to it and the worms feed it to the tree at the bottom. Lazy on my part but the tree is bountiful.
The Dirty Gardener
It's great to see you have received a comprehensive reply to your question by @dirtygardener.
If you were looking for suggestions for the feed I can recommend Richgro 5kg Black Marvel Premium Fruit And Citrus Food or Yates 500ml Thrive Citrus Liquid Plant Food as a direct fertiliser which will get the nutrients where they are needed now. For sustained growth and health, you can use Osmocote 500g Fruit Citrus Trees And Shrubs Fertiliser or if you wanted an organic option Osmocote Plus Organics 3.5kg Fruit And Citrus Fertiliser. Both of these products are slow-release and feed for up to six months.
If you need further information about these products or had other questions, please let me know.