Is it safe to drink tap water in New Zealand?

No. i work in this area. There are standards but they are rarely enforced. Unless you are in a major city you should boil the tap water. Esp if you are in a rural area you definately should boil before consumption, using to clean teeth, wash salads or make ice. This also applies to many food places and wineries as they are on their own supplies and untreated. Tap water here is third world except in major cities.
But Blenheim is fully compliant now.

I’d love to believe all the above thoughts but I have concerns. We travel all over for decades and the safety of NZ water supplies–one of the most advanced countries in the world–never would have occurred to me. (I’m a Prof working in biomedical research at a major University). Then reading chit chat in Tripadvisor I decided to check properly. Bacteria (and likely viruses, due to widespread floridation) are almost certainly not issues. However, NZ MOH standards for protozoa are “not detected” at 80% of the time. 80!! (So one chance in 5 in getting protozoa in your tap water is the goal. Wow.

Checking many locations( primarily rural, where we will be travelling such as Greymouth, Picton, Blenheim, Akaroa etc) shows that they do NOT meet this low standard. I’ll skip the risk of crypto or giardia and buy some water. Amazing! See most recent stats for where you’ll travel @ http://www.drinkingwater.esr.cri.nz/supplies/suppliescompliance.asp

Particularly for children, elderly or immunosuppressed I’d suggest caution based on what the NZ government offers.
Kudos to them for being transparent and helpful. Buyer beware.

Tap water, yes, certainly. NZ has well-defined, strict standards about drinking water.

There was a brief period of concern recently post-earthquake in Christchurch, where the aquifers and supply line got damaged, but that has since been restored.

So short answer: yes, and longer answer: yes, except sometimes immediately after unforeseen disasters.

I’m living in a high populated area (at least for NZ’s standard) in Auckland, but the question goes for the whole country.

Yes. Except in exceptional circumstances, NZ tapwater is universally safe to drink. Standards exist to ensure that major contaminating organisms such as Protozoa and Cryptosporidium are removed or reduced to safe levels.

Auckland water comes from two mains sources. West Auckland and some other parts of Auckland receive water from the Waitakere ranges catchment. Other areas receive water from the Hunua catchment and via pipeline from the Waikato river. The Waikato water is heavily contaminated at source and is treated to achieve requisite standards.

Auckland water is among the “softest” in NZ and better than most available in major cities internationally. eg whereas car batteries usually specify distilled water for topping up it is safe to use Ak tap water for this purpose.

There are variations between regions but these are mostly related to the nature of the water source. All NZ tap water is chlorinated.

About half of all NZ tapwater is fluoridated, with exceptions where local action has seen removal of fluoride from the water. Some consider fluoridation a negative measure with respect to health. The majority opinion sees it as a positive. The debate continues.

Drinking water standards for New Zealand:

The following document was found by following a series of links starting at the one provided by Mark Mayo in his answer. This is a 136 page PDF which provides extensive details.

NZ Ministry of Health – “Drinking-water Standards for New Zealand 2005 {Revised 2008} This is part of the NZ Ministry of Health online library.

NZ Ministry of Health library – water related items. A very large & useful resource:
140+ documents related to water with many of direct relevance to drinking water issues .

Only a few of the titles as a sample:

• Resources for the drinking-water assistance programme – numerous papers, including:

  • Treatment options for small drinking-water supplies
  • Sustainable management of small drinking-water supplies : Optimisation of small drinking-water treatment systems
  • Sampling and monitoring for small drinking-water supplies
    Water safety planning for small community water supplies : step-by-step risk management guidance for drinking-water supplies in small communities.

• Annual report on drinking-water quality
  Estimation of the burden of water-borne disease in New Zealand
  International standards for drinking-water
  Guidelines for drinking-water quality (World Health Organization.)
  Development of drinking-water standards for New Zealand
  Drinking-Water standards for New Zealand 1995
  Drinking-Water standards for New Zealand
  Guidelines for drinking-water quality : first addendum to third edition
  Drinking water quality : problems and solutions (Gray, N. F.)
  Provisional microbiological water quality guidelines for recreational and shellfish-gathering waters in New Zealand
  Community water fluoridation
  Cost benefit analysis of raising the quality of New Zealand networked drinking water
  Draft protocols for the public health grading of drinking-water supplies
  Pathogens and pathways, and small drinking-water supplies
  …
  And many more ……

Chlorination:

NZ water chlorination code of practice

Ministry of health notes on Protozoa and Cryptosporidium removal from tapwater

Fluoridation:

NZ Ministry of Health fluoridation page

Source Wikipedia

• The use of water fluoridation first began in New Zealand in Hastings in 1954. A Commission of Inquiry was held in 1957 and then its use rapidly expanded in the mid 1960s.[96] New Zealand now has fluoridated water supplied to about half of the total population.[97] Of the six main centres only Christchurch[98] and Tauranga[99] do not to have a fluoridated water supply. In 2013 a Hamilton City Council committee voted to remove fluoride from late June 2013.[100] A referendum was held during the council elections in October 2013 with approximately 70% of voters voting for fluoride to be added back into the water supply. The mayor and most councilors have indicated that they will support the outcome of the referendum.[101] In a 2007 referendum about half of voters in the Central Otago, South Otago and the Southland Region did not want fluoridation[102] and voters in the Waitaki District were against water fluoridation for all Wards.[103] Ashburton and Greymouth also voted against fluoridation.[104]

NZ anti-fluoridation campaign

NZ national newspaper discussion on Fluoridation

USA EPA comment

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Brief update September 2016:

We recently experienced “exceptional circumstances” in a small NZ town over a period of several weeks. The town supply came form bore water and was not chlorinated (to my great surprise). Contamination occurred (source when found was mentioned in the news) and several hundred people became ill, a few seriously so. It took several weeks to locate the problem wand water had to be boiled during this period. This is a very rare occurrence.