Specialists on statistics warn a proposed law change poses the threat of official data being corrupted by political meddling and unregulated sharing.

By Phil Pennington of rnz.co.nz

They are alarmed the new legislation will let the Government Statistician delegate their currently tightly-corralled powers to others, and that it fractures the longstanding constitutional divide between statistics and government policymaking.

"It is a sea-change," said constitutional lawyer and former Labour Prime Minister Sir Geoffrey Palmer.

Government agencies other than Stats NZ use statistics in policymaking, but at present don't get to decide themselves what to collect and how.

Former chief statistician Len Cook has made submissions to select committee and written media commentary condemning the new bill, and said people in power knew the influence stats had.

The idea the powers could be "transferred in full to other people, with no controls, with no limits" was "unusual in the world" and posed risks, Cook said.

In the late 1980s, when inflation was running at around the rate it is now, the government wanted to raise the tax on tobacco - and approached him about it, Cook said.

"Michael Cullen as Associate Minister of Finance requested the government statistician to remove tobacco out of the CPI and the government statistician said, 'I'm not going to do it'.

"I personally did tell him to get lost in somewhat trade union language."

He knew then he had the legal backup to guard his independence, Cook said - and believes the new Data and Statistics Bill risks that.

"The moment you transfer the authority and all the powers of the government statistician to someone who is more directly connected to a minister, and themselves has a role in keeping ministers content, is a very different thing."

The country's longest-serving statistics minister - most recently in 2014, National's Maurice Williamson - said no one would think multiple agencies printing money was a good idea, and the same applied to statistics.

Their importance was seen right now, when no one could question the inflation stats that had persuaded the government to keep the fuel discount in place, he said, recounting how the law had helped him protect statistics when he was minister.

"I had a number of cases where ministers in our government would call me and say 'look, what's happening with this labour force survey tomorrow - are we going to be in trouble with this?'."

They would ask him to sneak a peek, and he would say "'No, I can't'."

"And the government statistician, with the legislation the way it's worded, they could just stand there in front of their own minister and say 'minister, even if I wanted to give you these numbers, I can't'," Williamson said.

The Data and Statistics Bill is due back before Parliament soon, after passing its second reading in May.

Sir Geoffrey said the bill in section 16 underwrote the chief statistician's independence, but in section 44 gave the minister power to tell them what to do.

The delegation of powers appeared aimed at gathering lots more data.

"Hoovering up all the information that exists in the public sector does raise spectres of undesirable surveillance of the population as whole", whether that was the intention or not, Sir Geoffrey said.

There had not been enough scrutiny, he added.

ACT leader David Seymour said the party would raise the criticisms with Statistics Minister David Clark in the next stage of debate over the bill.

"Has he really thought through the importance of the chief statistician being independent?" Seymour asked.

Cook said the bill widened who could gather and disseminate data, risking unregulated data-sharing between agencies, in contrast with sharp boundaries in Australia and the UK, where he also served as National Statistician.

These other countries' laws delineated which parts of government got different types of information, for example for service delivery, for general statistical purposes, or for surveillance and enforcement.

"You need to be very careful when you're crossing those domains," Cook said.

Williamson said the 1975 Act was no impediment to technological advancements - as asserted by the new bill - and highly regarded.

"What you don't want is to weaken the structure of the system so that somebody can slip that level of corruption through the gate," he said.

"What you want is that robust steel wall of protection."

The select committee has recommended an extra protection to require the chief statistician to spell out how and why they are delegating their powers.

"We consider that the Statistician would be highly motivated to take care when considering possible delegations," its report said.

"This is because the Statistician would be responsible for any actions taken by a person acting under delegation."

Statistics Minister David Clark declined to comment and referred RNZ's queries to the chief statistician.