My 6 year old lime tree leaves are going brown on the underside. We have had lots of rain in south east Qld.

Thank you for providing photos of your lime tree.

The damage on your citrus leaves is caused by the Citrus Leaf Miner (CLM) pest. The adult is a tiny nocturnal moth that lays its eggs into young citrus leaves. A tiny caterpillar hatches from the egg and feeds on the inside leaf tissue, creating tunnels as it feeds, distorting the foliage as it goes to the edge, rolls the leaf over and then pupates. The pest is active throughout the warmer months. 

For control, prune off the affected leaves, then as a preventative, spray every 7-14 days with 'Yates Nature's Way Citrus & Ornamental Spray (Pyrethrum Plus Oil Pest Spray)'.  Click here for more info on this product: