The same survey found a quarter of building and construction work in Auckland was done under the table.
Earlier this year Silverdale-based plasterer Konstantine Malioutine was sentenced to eight months home detention for not declaring $500,000 in cash jobs.
What's more, points out tax adviser Terry Baucher of Baucher Consulting, you're taking a very short-term view by paying cash to a tradie.
"What happens if the work is defective?" he asks. "The builder might say: 'It was done for cash, what do you expect'?"
For the past couple of years I've had a rule of paying with eftpos in small stores. I hope in some cases this leads to the owners paying tax. It riles me to see a shopkeeper drop my cash in the till and not ring it up.
Recently a dairy owner asked me if I had cash when I presented my eftpos card to make a payment.
"So that you don't have to pay tax?" I asked, but there was no response.
Sadly, I'm a little deluded. That's because I'm told there is no guarantee small store owners are actually paying tax on their eftpos takings.
It means, however, if the IRD looks into their takings it can estimate how much cash isn't being declared by benchmarking the business takings against others in the same industry.
The rise of Airbnb is another area people could be caught out.
Owners who let rooms in their own homes sometimes think they come under the IRD's boarding rules, which allow a certain amount of tax-free income.
But Airbnb income on room rentals is taxable because it's a business.
The IRD says: "If you get money from renting out your house, a room, a caravan or a sleepout for any time at all - it's income."
The IRD has the power to get the names and addresses of all hosts and cross reference them against each individual's tax return.
Logically, it's personal choice if you want to mug someone for their wallet or break into their home. Getting cash for work and not paying tax on it is a crime.
Tax evasion does have an effect. I like living in a country with a half-decent education system, pothole-free roads, a medical system to fix me up if I'm broken, and so on.
None of these are perfect - but they need money to run and that dosh has to come from the rest of us if cheats don't pay.
One argument trotted out in the Facebook discussion was why should the rest of us pay tax if multinational companies find ways to pay next to nothing?
The argument is that the likes of Apple, Google and others use legal approaches to avoid paying tax on their earnings from New Zealand.
But that's up to the Government, the IRD and international agencies to sort out.
The fact someone else is not paying tax doesn't make it okay for the rest of us to do it. It's a race to the bottom if you look at it that way, my IRD contact says.
Some property investors also fail to pay tax on their earnings. It's not just investors, owner occupiers can be taxed on sale profits if they're buying, doing up and selling repeatedly. The capital gain on this sort of renovation is taxable as income.
The IRD is more on to tax evasion than the public believes. Baucher cites one client who had to prove his innocence. The client's relatives were coming to New Zealand to visit and he suggested they transfer their spending money in pounds to his credit card to use over here.
"The IRD picked up on this transaction and wrote to him," says Baucher.
"The IRD matched a credit card transaction from an overseas registered card on to a New Zealand tax resident."
Baucher also points out the 44,000 Trade Me members that the IRD chased. As a result of that campaign, traders paid up $3 million in undeclared tax.
