Thanks for your question
Yes, you need to declare the New Zealand income as well as all UK income as you will be treated as resident in the UK for tax purposes for the tax year 2012/2013, as you did not complete a full tax year abroad. (you may have spent more than 183 days not living in the UK, but as a UK citizen, to attain non UK residence, and subsequently non taxation on income earned abroad, you have to achieve at least one full tax year out of the UK which you did not )
Don't forget to also show the NZ tax suffered - as we have a double taxation agreement with New Zealand, and this will ensure you do not pay more tax then you need to.
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