Almost 6000 prostitutes were working before the Prostitution Law Reform Act was passed in June 2003, says a Ministry of Justice report.
The report, The Nature and Extent of the Sex Industry in New Zealand, is intended to set benchmarks so that the impact of the act can be assessed in future.
No data is available for after the act came into force.
That assessment will begin next year.
The report says the findings may not be accurate.
"Any attempt to establish the size of the sex industry must be viewed with caution.
"It is an industry where much of its activity has been hidden, and the non-regulated and fluid nature of the industry means than any estimate will simply be an indication of actual numbers," it says.
Justice Minister Phil Goff said the report might underestimate actual numbers.
The sources of information were police and the Prostitutes Collective.
The collective estimated that 70 per cent of sex workers were in massage parlours, 20 per cent in escort agencies and 10 per cent worked privately.
Transgender sex workers were estimated at 5 per cent of the total, although they made up about 50 per cent of those working on the streets.
The survey estimated 20 per cent of street workers and 8 per cent of escort agency workers were aged under 18.
Massage parlours were considered to be high-risk places for under-age workers because they could be more easily unmasked, but false identity documents were sometimes used.
Police said they were aware of cases of exploitation.
In one instance, a parlour charged clients $145 and the sex worker received $45.
Some sex workers had been given free drugs to get them "hooked" and were then forced to work to supply their habit.
Paying for it
* Of 383 sex businesses, 189 were massage parlours, 101 were escort agencies and 93 were rap/escort parlours.
* Of those, 243 were in the Auckland City police district. No other police district had more than 25.
* Of the 5932 sex workers identified, nearly half worked in massage parlours.