Stonewalling of requests through the Official Information Act also fed an increased perception of corruption, according to the Transparency International's head Suzanne Snively.
Politicians, journalists and the Public Service Association were also concerned with how the Official Information Act was working and its role in New Zealand's lowest dip in the CPI rankings since 1998.
Usually ranked in the top three least corrupt countries and often featuring in first place, New Zealand was ranked fourth out of 167 countries in the 2015 Index, its second consecutive drop after falling from No 1 in 2013 to No 2 in 2014.
Denmark was ranked first in 2015, followed by Finland and Sweden.
"There are a number of things which came up in the middle of last year such as the Oravida affair, the SkyCity affair, the Saudi affair, all of those are areas where the lack of transparency gave the impression things might not be as good as they could be," Ms Snively said.
She said the loss of New Zealand's reputation for being a country of integrity could be devastating for the economy and New Zealanders needed to make the effort to take part in the country's democratic processes and conversations to stop this from happening.
"It's a strong signal to us that we need to wake up.
"The one asset New Zealand has compared to other countries is our integrity and it's a very valuable asset to us. Other countries have things like mineral resources or a well-developed labour market, in New Zealand we have a very hard working population and to bring the returns to our economy, which we deserve because of how hard everyone works, we need to preserve our integrity."
Justice Minister Amy Adams said while the drop in the rankings was disappointing the report did reaffirm NZ was still one of the top countries in the world for low corruption.
"Our ranking reflects the good systems we have in place for investigating and exposing corruption in the public sector. What's more, we've demonstrated a zero-tolerance for corruption and bribery," Ms Adams said.
She said New Zealand was ranked across seven different categories, and was placed in the top two in five of them.
"While the slight slip in rankings to fourth place is disappointing, the Government has strengthened our anti-corruption measures and enhanced transparency since the underlying surveys for this index were undertaken, which we would expect will have a positive impact next year."
"The National-led Government takes corruption seriously and works hard to protect New Zealand's clean and transparent reputation," Ms Adams said.
The last time New Zealand was ranked fourth was in 1998 and from 2009 to 2013 it was ranked No 1.
Ms Snively said due to the way data was collected for the CPI before 2012 it was difficult to tell what caused the 1998 drop and subsequent rise back to No 1 in 2009.
In New Zealand seven different sources were used to measure the country's corruption, transparency and accountability in the public sector.
New Zealand was declining in rank in areas including order and security, fundamental rights and civil justice, lack of constraints on government powers and criminal justice,
absence of corruption, regulatory enforcement and open government, Ms Snively said.
Environmental governance was a particular area of concern, particularly New Zealand's levels of greenhouse gas emissions and water quality.
"We've got a strong level of environmental governance when it comes to protection of biodiversity, but we need to be working a lot more openly and assiduously around our water quality.
"While by most measures New Zealand is right up at the top, when it comes to our environmental sustainability we're at the mid point or below.
"I think that comes as a surprise for people because for years we've called ourselves 100 per cent pure, but that's one of the areas we've fallen."
Another area where New Zealand had fallen was in open access to information, with the stonewalling of Official Information Act (OIA) requests a concern, Ms Snively said.
Recent allegations of OIA abuse by government agencies by journalists who say they were charged hundreds of dollars in fees to have their OIA requests accepted could also have consequences for New Zealand's ranking in 2016 if nothing was done to address the issue.
Labour leader Andrew Little said there ought to be concern that New Zealand had slipped in the rankings for the second time in two years.
"This is not good. We've traded on our high ranking on that index for a long time. It's important internationally commercially and with our interaction with other governments," he told the Herald.
"Things like the Saudi sheep deal and the Oravida affair won't have helped but my own view of it is this Government plays it fast and loose with conflict of interest and although [the slip in ranking] is disappointing it doesn't entirely surprise me.
"What we repeatedly see is the Government sweeping things under the carpet and that gives the impression the government is willing to accept conduct that breaches conflict of interest rules."
Mr Little said in order to regain the country's top place in the CPI the Prime Minister, when faced with issues like the Saudi sheep deal or the Oravida affair, needed to take quicker action in either standing down ministers or opening independent inquiries into the matter.
The New Zealand Public Service Association also urged the Government to take the index seriously.
"While our members work extremely hard to maintain an open and impartial public service, the Government's been complacent about New Zealand's reputation," said PSA National Secretary Glenn Barclay.
Mr Barclay said he was also unsurprised by New Zealand's slip.
"Journalists and members of the public are reporting increasing manipulation of the Official Information Act, with delays and demands for payment becoming commonplace.
"The secrecy around the Trans-Pacific Partnership agreement, the murky process around the Auckland Convention Centre deal and Serco's handling of Mount Eden Prison have made things worse."
The Public Service Association is calling for greater transparency in the awarding of contracts, including full public consultation where services are privatised.
Journalist Nicky Hager, whose house was raided in 2014 by police in the hunt for the identity of the hacker Rawshark for Mr Hager's book Dirty Politics, also cited the "gradual undermining of the Official Information Act" as a reason he felt New Zealand would, and would deserve, to fall further down the CPI in future.
"Corruption doesn't happen because we have a corrupt population," Mr Hager said. "Corruption happens when there is too much secrecy, too much politicisation of public institutions and that's what we've been having in recent years.
"In my experience, if the Government wants to cover up something it's done, they almost always get away with it under the Official Information Act now."
The Corruption Perceptions Index (CPI) has been released annually since 1995, when New Zealand first topped the list as the least corrupt country in the world.
The CPI measures the perceived corruption of 168 countries' public sectors, due to the difficulty of measuring absolute levels of corruption.
Countries are ranked on a scale from 100 (very clean) to 0 (highly corrupt) based on external surveys and assessments from 13 international organisations.
Transparency International aggregates the data from these surveys to calculate its scores out of 100.
New Zealand was given a score of 88 in 2015, down from 91 in 2014.
• Denmark
• Finland
• Sweden
• New Zealand
• Netherlands
• Norway
• Switzerland
• Singapore
• Canada
• Germany
• Somalia
• North Korea
• Afghanistan
• Sudan
• South Sudan
• Angola
• Libya
• Iraq
• Venezuela
• Guinea-Bissau
What is Transparency International?
A non-partisan organisation tasked with reducing corruption. It works with governments, businesses and laypeople to research and gather data and perceptions about corruption and transparency. It releases regular reports about its findings.
It is generally well-regarded but has faced some criticism.
Where has NZ failed in its view?
Oravida scandal: Then-Justice Minister Judith Collins has dinner with bosses of the New Zealand milk company, of which her husband is a director, while on a taxpayer-funded trip to China in October 2013. In March 2014, details of the Oravida dinner emerged and Labour called for her to resign because of a conflict of interest.
Saudi sheep deal: $11.5 million of taxpayer money was spent on a single Saudi Arabian farm, including sending 900 pregnant breeding ewes to a disgruntled Saudi farmer, Mr Hamood Al Ali Khalaf, in 2015. The nature of the deal struck between Foreign Affairs Minister Murray McCully and Mr Khalaf was brought under scrutiny, with those opposed to the deal saying it amounted to bribing Mr Khalf to make trade between the countries easier.
SkyCity convention centre: When the government paired with SkyCity to build a convention centre in Auckland with the intention of attracting business to the country, questions were raised about how close the relationship was between the Prime Minister and SkyCity, dating back to the mid-2000s.
How does it measure transparency?
In New Zealand, surveys from seven different sources are used to find its ranking.
Two of these are open for anyone to look at, they are:
• Bertelsmann Stiftung's Sustainable Governance Indicators 2015 survey
• World Justice Project Rule of Law Index 2015
To look at the other five, you need a subscription. They are:
• Economist Intelligence Unit Country Risk Ratings 2015
• Global Insight Country Risk Ratings 2015
• Political and Economic Risk Consultancy Asian Intelligence 2015
• Political Risk Services International Country Risk Guide 2015
• World Economic Forum Executive Opinion Survey (EOS) 2015