I have room for one fruit tree in my garden - I already have a lemon tree and a kaffir lime

Its a sunny spot against a fence

Any suggestions for a fruit tree that doesn't require too much maintenance?

What about an espaliered apple tree - have seen one down our road and it looks stunning!!

A Hawera Plum or crab apple or persimmon.

In the Auckland area I would say either plum or feijoa. Both fruit well and require little intervention.
With regard to plums I like Luisa and with feijoas I would pick Apollo.

Along the lines of what Helen said, if you want low maintenance - in Auckland you'd do well to avoid a peach. The humid weather can make them quite a bit of work. I agree with a feijoa - you can just plant it and either let it go into a bush with some fruit on the edges, or if you want a lot of fruit, do some appropriate pruning and shaping in the first few years and you should be good. Feijoas are pollinated by black birds, so need to be fairly open in the middle so there is room for the birds to get to the flowers and do their job. They're also about as low maintenance as you can get wrt disease, just throw a bag of sheep pellets around the base twice a year and that should do you for feeding. Feijoas can self pollinate, but often do better with another variety close by.

Plums grow well in Auckland, but do need careful shaping in the first 2-3 years. There are many online guides as to how to do this. I planted 5 plum trees a few years back and they were all fruiting well the following year. I was pretty pleased with that given that after 3 years the peaches still were only managing 1-2 fruit each. I planted Black Doris, Luisa, Sultan, Omega and Billington.

If you're only planting one plum, then you'll need to look for one that is self-fertile - such as Luisa - as many are not.