Bill English and Jacinda Ardern square off in their first leaders' debate on Thursday night.

As Prime Minister Bill English and Labour leader Jacinda Ardern clashed during the first leaders debate, we fact-checked their claims.

We pulled out a few key assertions made by either candidate, put them in context, and checked them for accuracy.

Here's what we found.

Michael Bradley The party leaders had very different perspective on some of the facts at play in their first debate.

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PRODUCTIVITY

Questioned on New Zealand's productivity growth or the growth in the hourly output of Kiwi workers, English claimed it had "grown pretty well in New Zealand" while Ardern shot back that "productivity has flatlined at best in New Zealand".

Productivity growth is important because - in the words of the New Zealand Productivity Commission - "the higher the productivity of a country, the higher the living standards that it can afford and the more options it has to choose from to improve wellbeing."

So who's right here? A report from JB Were Investment Strategy Group released earlier this month (and referenced by moderator Mike Hosking during this exchange), claimed New Zealand has been in "a productivity recession since 2012" and "our economy has relied on more people, working more hours" to achieve economic growth.

This backs up the findings of an OECD report released in June, which found that although the economy was performing well "labour productivity is well below leading OECD countries, restraining living standards and well-being."

WAGES

After Hosking asserted that "under you [National] the wages have gone nowhere," English responded by claiming "wages have gone up at twice the rate of inflation". English backed up his assertion by pointing to increases in superannuation, which are tied to the average wage.

Under National, the average wage has increased from about $25 per hour in 2009 to $30 an hour in June this year.

Annual increases in the Labour Cost Index (LCI) - another measure of wages - have mostly been above inflation for the past five years. However, in the 12 months to June the LCI and the CPI (inflation) both increased at 1.7 per cent. This means that real wages did not increase in the 12 months to June this year.

HOUSING

While housing is an election priority across the board, Ardern described the issue as an ongoing crisis while English said the market is "flat to falling off".

So how about that flat to falling property market?

In the past year the the rate of growth in property values has slowed. The value of the average house increased by 6.4 per cent in the year to July, according to QV. In Auckland values grew at 5.3 per cent. In the 12 months before that, values were up by 14 per cent nationally and 16 per cent in Auckland.

Meanwhile Ardern said, "we've got the lowest home ownership in 60 years".

Have we? It didn't take long to fact-check this one: yes, we have.

Figures released by Statistics New Zealand in its Century of Censuses report showed the percentage of households that owned their own home had dropped to 64.8 per cent by 2013, the lowest rate since 1951 when it was 61.5 per cent.

Ardern also made a passing reference to the average deposit in Auckland being $150,000.

And that's certainly plausible, given the average house value in Auckland City, according to the QV House Price Index, is more than $1 million. A 10 per cent deposit, therefore, is around $100,000 (that's about 8156 brunches of avocado on toast, if you're wondering.) Of course not everyone can purchase a house with a 10 per cent deposit.

APPRENTICESHIPS

The leaders had starkly different views in this area.

According to English, every young person who wants an apprenticeship can get one.

"It's great to be able to talk to 18 year olds … they're saying to me companies are competing for us," English said.

"We're in the biggest construction boom New Zealand has ever seen. For 18 year olds leaving school this is the best opportunity in a generation to be able to see steady work and rising incomes."

Ardern had a rather different version. She said as soon young people get out training they can't find a job because employers say it's too expensive.

"I went to MIT today there's a number of carpenters going into training at the moment," she said.

"The difficulty they find is then getting those apprenticeships and that's why we want to work with employers, subsidise them to take them on, and that will help."

This is classic fuzzy territory for politicians. Each drawing on their own anecdotal experiences to assert totally different versions of the truth.

The Careers NZ jobs database suggests carpentry trainees have a "good" chance of getting a job "due to a building boom". Average earnings would be $18-$38 per hour, it says.

According to the Ministry of Business, Innovation and Employment's outlook to 2019, the strongest employment growth is expected in construction and related industries.

WATER

Ardern said her party's proposed water tax is motivated by a need to clean up our rivers.

During the debate she said: "60 per cent of our monitored waterways you can't swim in any more."

Is that true? Well, it depends on who you ask.

Essentially, the Government's "swimmable" standard has shifted - whereas the previous measure for water deemed to be safe to drink and swim in was 260 E coli units per 100 millilitres, that has now been pushed out to 540 units.

That new approach to measuring swimmability was launched as part of the Government's Clean Water Package in February this year. Headlines at the time highlighted confusion around the standards. As environment reporter Charlie Mitchell said: "There are slippery politics around the word swimmable".

So, which definition was Ardern referring to? We can't be sure, but Greenpeace's executive director, Russel Norman, has repeatedly quoted the same number in recent times.

That number also appeared in a Ministry for the Environment indicator report released in 2013, which found 61 per cent of monitored waterways in New Zealand were "poor" or "very poor" quality.

In April this year, the Ministry for the Environment put out a report called Our fresh water 2017, which found plenty of problems with waterways, including nearly three quarters of native freshwater fish species are threatened by or at risk of extinction, as well as a third of native freshwater invertebrates and a third of native freshwater plants.

But it also found between 65 and 70 per cent of rivers would be deemed swimmable under the new standards. Meaning just 30 to 35 per cent would not.