Can I put that down the kitchen sink?

We've all heard the warnings about wet wipes and fatbergs but the kitchen sink is often where complacency creeps in. A splash of pan oil here or a rinse of leftover sauce there might seem harmless at first.

According to plumber Marc Brouwer, who has worked across Australia and New Zealand for 22 years, kitchen drain blockages are an everyday callout.

"It can range from the original plumbing that may have been installed, like the wrong pipe sizing. It can be due to old pipes... Then in most cases it's all just self-inflicted, which is pouring oil down the sink."

Hot oil and meat fats are the main culprits, Brouwer says. When they hit cold water or are left without hot water running, they solidify - building up over time and putting a strain on wastewater treatment plants and septic tanks. Starchy foods (like rice and pasta) and dairy-heavy liquids can also cling to pipes.

Old, galvanised pipes are especially prone to catching residue as rust creates a rough surface inside the pipe, he says.

If there's a small amount of residual oil, Brouwer says run hot water - but ideally, don't pour oil down at all. Wipe pans with a paper towel first and bin the grease. Even with hot water, fats can cause blockages, including in outside pipes or gullies.

Professional organiser and housekeeper Rachael Quinn agrees. Apart from the odd grain of rice, only water and detergent should regularly go down the sink, she says.

"The more that we put down there, the more it's going to clog up over time, and of course a lot of the food we're having these days is highly processed, so there's a lot more things that make it just stick."

Medicines should never go down the drain either.

"It's a big fat no. What you should do is gather them all up and actually take them back to a pharmacy. They will dispose of them correctly."

"Realistically, you shouldn't have a waste disposal, to be honest ... they cause more problems than what they're worth," says Auckland-based Brouwer.

Limit the unit for "minimal scraps", he says, like broccoli remnants or lemon or orange peels, which are acidy.

Eggshells, rice, pasta, coffee grounds and stringy vegetables like celery are all risky, he says. Rice and pasta expand; eggshells can turn into gritty sludge that lines pipes. Bones are also off-limits.

Quinn, who has a disposal unit, avoids onion, garlic, and banana peels, and never puts heavy fats down nor tea bags. She reckons lightly oiled pans are fine to rinse in the unit along with boiling water, but shallow- or deep-frying oil should be disposed of appropriately. (Check your local council for guidance.)

To keep units fresh, she suggests running lemon wedges and ice cubes through with water about once a week.

For expired sauces: Oil-based dressings belong in the bin, she says. Chunkier sauces can go in compost or, in small amounts, the disposal unit.

On top of that advice, the guidance for septic tanks by Auckland Council widens to include antibacterial soap and cleaners, chlorine and coffee grounds.

Septic tanks should be pumped every three years and fitted with an outlet filter to prevent solids entering the disposal field, the council says. And, for these households, it advises against installing waste disposal units too.

Small amounts of everyday household cleaner are usually fine, but repeated or heavy chemical use isn't recommended, says the country's largest water industry body, Water NZ.

Baking soda and vinegar are commonly recommended as septic-safe cleaning alternatives when diluted.

Even water-based paint should not be rinsed down sinks, because pigments and binders can pollute waterways or interfere with treatment systems, Water NZ says.

Instead, wash brushes in a container and let solids settle in a container, pour off clear water onto soil (not storm drains), and dispose of dried residue in the rubbish.

Quinn avoids heavy-duty chemicals altogether, noting they can corrode pipes over time.

Her DIY solution is letting a paste of baking soda sit for 10 minutes in the drain and then washing it down with boiling water. (She doesn't wash it down with vinegar as that neutralises it.)

"I would certainly suggest once a week to pour two to three boiling water jugs full down your kitchen sink just to push everything through."

At worst, fats and wipes (which also shouldn't be flushed) combine to form fatbergs in public wastewater systems, Water NZ says.

At home, blockages can cause wastewater overflows - bringing odours, mould and bacteria like E. coli. But generally, the environmental impacts of household-scale blockages are typically small, Water NZ says.

Medicines can disrupt wastewater treatment processes, enter groundwater or waterways and contaminate septic systems.

For septic systems, blockages or chemicals can cause untreated effluent to surface or back up. Strong chemicals, paints, medicines, and oils can kill the bacteria needed for septic tank systems to break down the waste.