An avocado can sit on a healthy, well-fed tree for weeks with skin that looks exactly right, and never once go soft. This confuses a lot of first-time growers, because almost every other fruit tree signals readiness by softening on the branch. Avocados don't work that way. They belong to a small group of fruits that will not ripen while still attached to the tree; the tree itself actively holds the fruit in an unripe, storable state, no matter how long it's left hanging there.
The tree does this by keeping the fruit's own ripening response in check. Avocados ripen through ethylene, a gas the fruit produces itself once conditions are right, and while a fruit is still connected to the tree, the tree keeps that response suppressed. Cut or twist the fruit free, and that suppression stops. The fruit is now on its own supply, and within a day or two it starts producing enough ethylene to trigger the enzymes that soften flesh, break down starches into sugars and develop the full flavour a ripe avocado is picked for. Nothing needs to be added. Picking the fruit is the trigger.
Picking the fruit is the trigger.
Because there's no colour change to watch for on the tree, especially with the thicker-skinned varieties that stay dark green even fully mature, growers rely on two simpler cues instead. The first is size and roundness relative to how the fruit looked earlier in the season, since a mature fruit fills out and loses its slightly angular young shape. The second, more reliable test is the stem cap: flick or gently twist the small woody nub at the top of the fruit, and if it comes away easily and shows green or pale yellow underneath rather than dry brown, the fruit is ready or very close to it. Avocado is a full-sun tree with moderate water needs and it's rated an advanced crop in this site's own plant records, largely because of how slow and easy to misjudge this stage is; a home tree can hold ripening-ready fruit for months, since New Zealand's common garden varieties don't all mature and drop together, so one tree is usually picked gradually across a long window rather than all at once.
Once picked, ripening is easy to control because it runs on that same ethylene signal. Leaving a hard avocado in a paper bag on the bench traps the ethylene the fruit is giving off around it, instead of letting it dissipate into the room, and speeds things up noticeably. Adding a banana or an apple to that bag does more of the same thing, since both produce large amounts of ethylene of their own and effectively lend the avocado a stronger dose of its own trigger. A hard avocado left out at normal room temperature typically softens over four to seven days on its own; warm weather shortens that, and a cool room stretches it out.
The opposite trick works just as well. Refrigeration slows the ethylene reaction right down, which is why a firm, unripe avocado can go straight into the fridge and hold there for a couple of weeks with barely any change, ready to bring out and ripen on the bench whenever it's actually wanted. This is worth knowing at harvest time specifically, since a home tree carrying dozens of fruit at once will otherwise force a choice between eating avocado for a fortnight straight or losing the excess to overripeness; picking a few at a time and holding the rest cold in the fridge spreads the same harvest out much further.
None of this is a problem with the tree, and it isn't the same issue as a seed-grown avocado that's simply too young to fruit yet, which is its own separate wait covered elsewhere on this site. A mature, fruiting tree that won't ripen on the branch is behaving exactly as an avocado is supposed to: it's just waiting to be picked before it does anything at all.