If you're on a budget, you'll find that taking a freighter back to New Zealand from Fiji probably is going to cost you substantially more than flying, if you can find a ship plying that route going when you want to.
The cost of a freighter cruise is calculated on the basis of a daily rate, with most voyages priced between US$125 to $150 per day, plus fees and insurance.
If you're determined, there are several Web sites that provide information on freighter travel.
Earlier this year I looked into freighter travel in the South Atlantic, including taking the RMS St. Helena from Cape Town to St. Helena and Ascension. I couldn't fit it in my travel plans. The ship's last voyage was scheduled this month with the opening of St. Helena's airport. But that opening has been postponed, according to a report this week: http://www.dailymail.co.uk/news/article-3622981/The-250m-island-airport-jets-t-land-windy-guess-aid-money-paying-it.html?ITO=1490
Now, the sailings have been extended through September. But I've made other plans.
Hope this helps. Have fun in Fiji!
P.S. Check the Travellerspoint guide to Fiji.
[ Edit: Edited on 04-Jun-2016, at 04:27 by berner256 ]
thanks for your answer, has been very usefull!