It's tempting to do a squeeze to see what's going on under the skin. So tempting, that most of us do it.

The problem with avocados is you don't know what's under the knobbly purple and green skin.

So it gets a fingertip squeeze to feel for the right "give".

Feel that and there's a good chance this will be the soft pale green and yellow fleshed avocado wanted tonight - instead of a hard tasteless one or a brown, slimy mess.

Supplied A nightmare sight no avocado lover wants to discover.

Everyone does the secret squeeze. But think about it. Everyone also squeezed the one you just bought.

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And "everyone" isn't exaggeration. A survey reported by Hort Innovation Australia found that 97 per cent of avocado buyers admitted to squeezing them before buying them.

123RF A perfect avocado - but how do you know it will be before cutting it open?

Even worse, on average the avo buyers squeeze tested three times more fruit than they actually bought.

While avocado growers and retailers had worked hard on looking after avocados carefully before they got to shoppers, it turned out being exposed to shoppers was the most dangerous time for an avocado.

Lead researcher Professor Daryl Joyce said squeezing, or as he called it "compression events", generally caused enough damage to bruise the fruit flesh and it took about 24 hours for the bruising to develop and became visible.

NZ Avocado New Zealand Avocado offers this chart to help choose ripeness using skin colour.

"It has been found that shoppers typically apply compression forces ranging from 3 to 30 Newtons (N) to firm-ripe avocado fruit when assessing ripeness," Joyce said.

He said a "slight" thumb compression of 10 N applied to a firm-ripe fruit caused bruising to appear within 48 hours at 20degC.

But strangely buyers didn't link the problem of bruised fruit to handling by other people.

New Zealand Avocado chief executive Jen Scoular said avocado squeezing before buying was a problem in New Zealand

"It's not helping the quality for the next person."

Instead, the industry recommended shoppers use skin colour to guide them.

"If it is purple it is ready to eat. When it's purpley green, it has probably got another two days to be fully ripe, but you could still use it in a salad."

A black avocado which had lost its shine was probably going to be a brown disappointment.

Scoular said the avocado industry relied on supermarkets to tell buyers about colour and ripeness but that information wasn't always made available.

She said Australia had avocados sold in different trays depending on whether they were ready-to-eat-now, eat in 1-2 days or eat in 3-5 days. That had boosted sales but it was more work for retailers.

This was trialled in New Zealand a few years ago and worked well, but erratic sales had made it too much effort for retailer to keep going. On a sunny day, for example, sales of ripe avocados could be suddenly four times higher because everyone wanted a salad.

Another way to tell ripeness that horrified the avocado industry was pulling off the stem and looking into the hole.

The revealed the colour of the fruit inside.

But Scoular said doing this before buying, and leaving it, left a damaged avocado for someone else to buy, or to be wasted.

"The stem is part of the avocado. We don't snap the stalk of the broccoli off before buying it, or take the stalk off the apple," she said.