| The previous posts point to the likely problem, soil. Cate, it cloud be anyone of a number of problems. Yellowing leaves can mean a soil deficiency of some sort such as Iron, magnesium etc. For magnesium water in Epsom Salts. Be very sure if it Iron as applying Iron sulphate can make the soil too acid. The soil may well be too acid ie. the pH is too low. If you can get a pH tester from a friend or buy one from any nursery (I know there is a good one in Brunswick St. Fitzroy). They are not all that expensive, they last for years and every gardener should have one. Get the one with the powder and the liquid rather than the inaccurrate ones with the metal prong. It is also possible that you have planted it in a hard clay soil where the roots and the water cannot move out. That would result in a overly wet rootball which would kill any growing roots. Then again the rootball may be too dry through insufficient watering. Another tack would be to cut of an affected shoot and take it to a nursery where they can advise you.
Those old houses in Brunswick have soils with a long history. The previous owners could have dumped something unpleasant there. So you can see it could be anyone of a number of problems. Try Googling Citrus, soil and Australia and you will get an enormous amount of information to read. Also try googling iron and/or magnesium deficiency in citrus and check your plant with the illustrations. | 
