Is that big tree likely to fall down after the cyclone? Here's how to tell

Even as Cyclone Gabrielle loosened its grip on Aotearoa, trees kept falling - many are wondering whether trees near them might fail.

Over 48 hours, the ex-tropical cyclone brought down trees all over Auckland and further afield, bringing down power lines and damaging property and people.

Scott Geddes, arborist and director of Auckland Tree Services said more trees might come down this week.

"They've been blown around and sitting in sodden soil for a few days now," Geddes said.

He said while the ground may dry quickly in the coming days, tree root systems have been weakened by the wind.

"They're being blown in a different direction to what they have grown to withstand, so it might be we see a longer time period where things fall down - because of the initial weakness it is a bit of a delayed fuse.

"But I'd say we'll see a sharp tailing off if we get a few days of really good sun. I can imagine things should stop falling over, but there is damage that will take its time revealing itself."

If anyone was worried about a tree near them, "call an arborist, lickety split", he said.

Tree Fellas general manager Ryan Kneebone said assessments can take just 10 or 15 minutes and an arborist can be with you within 24 hours.

There were a few signs to look for when judging whether a tree was likely to fail.

If the surrounding ground looks different, with new bulges or cracks, that could indicate it was coming loose.

"Trees on a lean that weren't leaning before would be a sign that something isn't quite right - that maybe the roots are too wet, and they're not holding on, or they're not fused together that well," Kneebone said.

One of the biggest telltale signs a tree will fail was wobbling at the base, Geddes said.

Often, trees on boundaries lose some of their roots on one side due to earthworks in one of the properties. That's what happened at one Remuera site, causing the tree to topple almost exactly along the boundary line during the storm, Kneebone said.

"Most of the time when large trees fall over you can tell straight away if there was some soil disturbance, which mainly comes from development or incorrect planting."

Geddes said if the tree - big or small - was not wobbling at the base, or showing roots where it wasn't before, do not cut it down.

"One of the biggest issues is people cutting trees down when they don't need to," he said.

"Trees suck a whole lot of water out of the ground, so knocking trees over for the sake of it is a detriment."

It was important to check soil conditions. "If you can see puddles or water, definitely that will be more susceptible to total failure."

But there was not always rhyme or reason - of the 100 or so trees Geddes inspected at one site on Tuesday, one still came down hours later, even though it showed no signs during the inspections.

Geddes, from Henderson, was up until 2am on Wednesday clearing trees from roads and homes.

"It's more total tree failure in one go than I've ever seen before and that's due to the fact the soil was already sodden from all the flooding we had."

Most calls to his nine-person team were "screamingly urgent", he said, with trees blocking roads or on houses. One on Tuesday night involved a gum tree that went entirely through a roof.

Since Gabrielle, poplars were the most common tree he has spotted having "keeled over", he said.

"They have a habit of getting obnoxiously big and heavy. They're not a great tree, the wood in them is reasonably weak and you see them tipping over outside of extreme storms. I couldn't tell you why."

Auckland Council regional arborists and ecological manager David Stejskal expects the risk of trees falling will reduce this week.

"While tree failures are rare, the combination of waterlogged soil and high winds can increase the likelihood, which is what we have experienced in recent weeks," he said.

If you see problems with trees on public land in Auckland, report it online so the council can investigate. An arborist should take a look at worrisome trees on private land.

"It is important to note that there are rules around the pruning and removal of certain trees, so it is always worth checking the council website before any work is done.

"While we understand people may be concerned about trees failing, it is important to remember that they do provide many benefits, including improved land stability and reduction of storm water runoff."

- This story was first published on Stuff

National Emergency Management Agency advice:

• Put safety first. Don't take any chances. Act quickly if you see rising water. Floods and flash floods can happen quickly. If you see rising water do not wait for official warnings. Head for higher ground and stay away from floodwater.
• Do not try to walk, play, swim, or drive in floodwater: even water just 15 centimetres deep can sweep you off your feet, and half a metre of water will carry away most vehicles.
• If you have evacuated, please stay where you are until you are given the all-clear to go home.
• If you don't need to evacuate, support those who do by staying home, staying off roads and staying safe.
• If you are not able to contact your whānau in the heavily affected areas go to Police 105 website and complete the inquiry form or phone 105 and remember to update if you reconnect through other means.
• Throw away food and drinking water that has come into contact with floodwater as it is often contaminated and can make you sick.
• If you are without power eat the food from your fridge first, then your freezer. Then eat the food in the cupboard or your emergency kit.
• People should stay up to date with the forecasts from MetService and continue to follow the advice of civil defence and emergency services.
• A National State of Emergency is in place for an initial period of seven days and applies to regions that have declared a local State of Emergency.