Anything could be causing that and unless we have details we won't know exactly what it's from

Hi Mike - what details do you need? Am in Sydney Australia. Cheers

Cheers to you too)

Lots of questions that will help.

Is that tree outside now? Has it been outside recently? What kind of fertilizer do you use? What kind of soil mix? What kind of water> pH of it? Is it soft or hard water? How often do you water? What temps is that tree use to? Have you checked the roots? Have you ever flushed the mix of dissolved salts? Do you use just tap water? Is that tree in how much sunlight?

Just a few to start.

Mike)

The leaves are showing a manganese deficiency. The symptom that a citrus tree produces when manganese is deficient in a leaf is as follows: green veins, AND WITH GREEN BORDERS along the sides of the veins, as show in the picture provided. This is rather common in new leaves. and most of the time the tree will take care of the deficiency on it own. The reason for the deficiency is that manganese is in soil in a very small amounts. The new leaves are growing faster than the tree's root system can collect enough manganese to supply the rapid growth. However, over time the tree catches up. You can foliar spray a manganese solution to quicken the process of reliving the deficiency if you wish,, but most people leave it up to the tree.