Overdue Census forms are still flowing in more than seven weeks after the March 5 official Census day.
Census general manager Carol Slappendel said more than 20,000 forms had been received in the fortnight since April 11, when collectors stopped going back to houses that had not returned forms.
"In the two weeks prior to April 11 we sent collectors back out in some areas to chase up outstanding forms, and in other areas we also mailed out reminder letters to households. This created a flow of mailed-back forms," she said.
"I'm very happy with that. The more we get in of these outstanding forms, the better."
Although online Census returns closed on April 11, Ms Slappendel said Statistics NZ would continue to give people "multiple opportunities" to file their forms on paper before prosecuting anyone. Any person failing to fill in a form can be fined up to $500 plus $20 for each day after conviction until the person completes the forms.
"We are just in the process of gathering in reports from our field operations," she said.
"If they have had refusals from individual members of the public, that will be documented. You can only mount prosecutions if you have sufficient evidence. Then we start a process of following up with many of those people.
"We do give people multiple opportunities to fill in their forms even through the prosecution process. We keep accepting forms right up to the very end, possibly into next year."
In all, the Census has collected 5.6 million individual and dwelling forms, including 1,937,823 filed online. Online filing hit the target of 35 per cent almost exactly, surpassing 30 per cent achieved in the 2011 Australian Census.
A "post-enumeration survey" is now under way, knocking on a randomly chosen 15,000 doors to estimate the Census "undercount" - the number of people who were not counted on March 5. Ms Slappendel said initial indications were that the undercount was similar to the last Census in 2006, when it was estimated at 81,000 people (2 per cent).
All paper Census forms are now being scanned at an Auckland facility. Processing of both the scanned forms and online returns has just started at a facility in central Wellington.
"It will take about 200 people working in two shifts, five days a week, for five months to process all the forms," Ms Slappendel said.
The first use of the Census data will be in October when Statistics NZ will announce the number of general and Maori electorates for the 2014 election, based on population growth and the Maori electoral option process which follows every Census.