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. - 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.
Black marvel is applied at 50 grams per square meter. You could also try some Yates 500ml Citrus Cure Zinc & Manganese Chelate at the prescribed rate on the packaging as the plant might be deficient in those elements. - MitchellMc
Mitchell has given sound advice regarding the Black Magic and also the zinc and manganese chelate product from Yates.
Yellowing of the leaves with green veins makes me think there may be a magnesium deficiency present. If the fertiliser and other treatments don't make any improvement to the tree's appearance, try good old-fashioned Epsom Salts (magnesium sulfate) at the rate of 1 dessertspoon dissolved in 9L (standard watering can) of water.
Citrus in pots need a couple of good soakings per week over the warmer months - water until excess drains out the base of the pot each time. Over the cooler months, do the finger test before watering - if the top of the mix is dry to the second knuckle, add water.
If the top of the mix is dry and the mix in the base of the pot very wet, then drainage is likely a problem. Increase the size and number of drain holes in the base of the pot and stand the pot up off the ground on pot feet or a triangle of bricks or similar so the drainage holes are clear and will allow excess moisture to flow out readily. A few holes around the perimeter of the base could be helpful but should not be necessary if the base is above-ground.
The other issue may be that you need a layer of coarse material in the base of the pot to facilitate drainage - blue metal, broken up pieces of terracotta pots ('crocks') or similar to a depth of 50mm or more depending on the size of the pot. - Noelle
Yellowing Leaves can also be caused from cold damp mixes, especially in winter. Lemons quite often go yellow then because they feel like it! They need to stay on the dry side through winter and an occasional foliar feed with seaweed will provide them with the nutrients they need.
Feed once a month as well, but you don't want to encourage growth in winter. I have noticed this a lot when the mix is staying too wet but they go yellow when their life cycle ends as well. Typically two to three years is a leaf span I believe, then they go completely yellow as the tree sucks back what nutrients it has left in its leaves.
Spring time is the key. Mid-August feed with good nitrogen and new growth should be good. - laidbackdood