Tight state funding has helped push New Zealand universities down to 15th place in a new world ranking.

The Universitas 21 list, which ranks national university systems rather than individual universities, once again puts the United States at the top of its 50 countries.

Switzerland, Britain, Denmark, Sweden and Singapore come next, with Australia steady in tenth place.

New Zealand universities have dropped six places in the past five years, from ninth place in the first list in 2013 to 14th last year and now 15th.

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The report, written by a group at Melbourne University, said New Zealand's decline over the five years was due mainly to falling behind other countries in the numbers of researchers per million people and in the proportion of people aged 25 to 64 with tertiary qualifications.

"The increase in the percentage of the population aged 25-64 years with a tertiary qualification has not matched the increase of 9 percentage points in the median value for our 50 countries," it said.

In the past year, the biggest drop in New Zealand's rankings is in the "resources" section, where we have dropped from 18th place to 23rd.

"In New Zealand, the reduction in Government funding as a share of GDP causes the rankings to fall five places to 23rd, back to the level in the 2015 rankings," the report said.

The rankings are based on a mix of measures, only some of which are adjusted for population. For example, 10 per cent of the overall ranking is based on "total articles produced by higher education institutions" - a measure easily topped by the United States simply because it is the most populous developed nation in the list.

Only 3 per cent of the overall ranking is based on "total articles produced by higher education institutions per head of population".

Other "output" measures, including numbers of researchers and percentage of those aged 25-64 with tertiary qualifications, add to give a total score for "output" worth 40 per cent of the overall ranking.

New Zealand's ranking for "output" actually rose slightly this year, from 21st place to 19th.

"Resources", where we dropped from 18th place to 23rd, includes state tertiary spending as a percentage of GDP, annual spending per student (not adjusted for GDP), and three other measures adding to 20 per cent of the overall ranking.

New Zealand's best ranking, second after the US, was for the educational "environment", including Government policies, university independence and a survey asking employers how well the education system "meets the needs of a competitive economy". This category also accounted for 20 per cent of the total score.

The final category, also accounting for 20 per cent of the total, is "connectivity", measuring the proportion of international students, articles co-authored with foreign academics, articles co-authored with industry researchers and similar items.

New Zealand's ranking in that category rose from tenth to seventh, including the second-highest proportion of international students after Singapore.

New Zealand's place also improved, from ninth to seventh, in a separate overall ranking adjusted for GDP per person. That list was topped by Serbia, Britain, South Africa, Denmark and Sweden.

Universities NZ executive director Chris Whelan said New Zealand had "an efficient, high-performing university sector that is struggling to maintain its place in the world against a backdrop of underfunding".

"It should be of concern to all New Zealanders that official statistics show that the NZ Government's expenditure on tertiary education institutions is below the OECD average," he said.

"Our universities, wānanga and polytechnics now sit amongst a range of countries we do not traditionally compare ourselves against, including Indonesia, Colombia, Mexico, Brazil and Turkey.

"However, the Government has the opportunity to put this right in its Budget later this month."

Universitas 21 Rankings

1. United States

2. Switzerland

3. United Kingdom

4. Denmark

5. Sweden

6. Singapore

7. Canada

8. Netherlands

9. Finland

10. Australia

11. Austria

12. Belgium

13. Norway

14. Hong Kong

15. New Zealand