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As per your letter - The last date you may travel to New Zealand is: 23 December 2019 - this means you need to arrive in New Zealand on or before December 23, and This visa expires 1 month(s) after your first arrival means you have one month after that arrival before the visa expires (and therefore need to leave the country).
So yes, you can arrive on (or before) December 23 and stay for 10 days. But you cannot arrive on December 24 or later.
Be aware that for traveling from most countries, NZ Time is ahead, so while you may leave your origin on Dec 23, you may end up arriving on December 24! Make sure you account for this both in travel time and time-zone adjustment.