Steel vs plastic water tanks: comparing the facts

Choosing between a steel or plastic (poly) water tank comes down to a handful of practical factors. This guide compares them, using evidence rather than general claims. There’s no universal winner. The right choice depends on your site, your budget, aesthetics and what matters most to you.

The short version

Cost: For Slimline and Modline tanks, the shapes we manufacture most, steel is often equivalent to or cheaper than plastic, even at smaller sizes. Round plastic tanks are generally cheaper upfront than steel.
Weight: At equivalent capacity, steel is often the lighter option, not plastic.
Customisation: Steel tanks are custom made to your dimensions and capacity requirements; plastic comes in fixed shapes and sizes.
Drinking water: Tanksalot’s steel tanks have a food-grade lining that meets AS/NZS 4020 for potable water storage; not all plastic tanks meet this standard, so it pays to check before you buy.
Warranty: Tanksalot’s steel tanks are backed by a 20-year corrosion and 10-year construction warranty with no pro rata reduction.
Fire safety: Steel is non-combustible and can sit against boundaries and dwellings; plastic has setback requirements.
Best fit: Steel for most urban or visible sites; round plastic for basic rural storage on a budget; concrete for large-scale or permanent in-ground applications.

Cost

Cost comparisons between steel and plastic tanks depend heavily on shape as well as material. Round plastic tanks are cheaper than an equivalent round steel tank, but round isn’t the shape most homes and urban sites need, and many people assume that price gap holds for every steel-versus-plastic comparison. It doesn’t. For Slimline and Modline tanks, the shapes we manufacture most, our steel tanks can be equivalent to or cheaper than plastic even at smaller sizes, especially once you factor in installation and connection costs. With our corrugated steel water tanks, you choose exactly where your plumbing connections need to be positioned to meet your requirements, and our team installs all the fittings.

Every tank we manufacture is tested for water tightness before dispatch, so it arrives with fittings in place and minimal connection work required by plumbers. That eliminates any risk of a tradesperson needing to cut into the tank on site, and it can significantly reduce plumbing material and labour costs compared to plastic tanks with fixed connections that may not suit your site.

Because our tanks look great, many buyers also avoid the screening or landscaping costs that plastic tanks often need to stay out of sight.

Weight and transport

It’s a common claim online that steel tanks are heavier than plastic. For our corrugated steel water tanks, that’s not accurate. A 5,000 litre plastic tank typically weighs in around 260kg, while our equivalent steel tank can weigh as little as 145kg.

We offer free delivery to metropolitan Auckland and freight our water tanks nationwide. Contact our team for a free freight estimate.

Shape and customisation

Plastic tanks come in fixed shapes and sizes. Our steel water tanks are custom made to fit your space. You simply specify the footprint, height, and connection positions before manufacture. Steel also holds its shape without bulging or bowing as it fills, something plastic tanks can be prone to over time.

Drinking water safety

If you’re storing water for drinking or household potable use, your tank needs to meet AS/NZS 4020, the standard governing materials in contact with water intended for human consumption. Our corrugated steel water tanks have a food-grade lining that meets this standard. The internal stainless-steel rods that ensure our tanks hold their shape are sealed with food-grade silicone, so every water-contact point meets the same standard. Our tanks also carry no BPA risk and don’t need UV stabilisers. Not all plastic tanks meet the AS/NZS 4020 standard, so it’s worth confirming compliance before you buy.

Warranty

Our corrugated steel water tanks are backed by a 20-year corrosion warranty and a 10-year construction warranty, with no pro rata reduction over the term. This means the cover doesn’t shrink year on year the way many plastic tank warranties do. Our full-term cover is uncommon in the New Zealand water tank market.

Fire resistance

Corrugated steel, sometimes called corrugated iron, is a non-combustible material, so our tanks can be placed against boundary walls or beside dwellings under NZ building rules. If your site is tight, or the tank needs to sit close to a structure, this is one of the more significant practical differences between the two materials. Plastic tanks are generally not fire rated.

Aesthetics

Our corrugated steel water tanks are available in eight Colorbond® colours designed to integrate with NZ residential architecture. Many water tank buyers find a corrugated steel exterior more visually appealing than a plastic finish, and it’s one reason plastic tanks are more often screened or landscaped to keep them out of sight, which can get expensive, fast. For customers wanting a unique tank, we offer custom colour sprays for an additional cost.

Environmental impact

Corrugated steel water tanks are fully recyclable at end of life, with the lowest overall environmental impact compared to plastic or concrete tanks. Sustained UV exposure is also a known contributor to plastics breaking down into microplastics over time, a growing environmental concern for both virgin and recycled plastic.

Concrete water tanks: a third option

Concrete is the third material used for water storage in New Zealand, though it shows up far more often in large in-ground and commercial installations than in everyday household setups. Concrete water tanks are either poured on site or delivered precast, and both routes involve real engineering and site work before the tank is in the ground.

Key things to know:

Very heavy. Poured and precast concrete tanks need truck access and often a crane, plus substantial excavation for in-ground installations
Long service life and highly resistant to fire and UV when engineered and built correctly
High thermal mass, which keeps stored water cool
Effectively permanent once placed. Relocating or removing a concrete tank later is a major job, so the decision needs to be right the first time
Can crack over time, particularly with ground movement, and cracks can be difficult and costly to repair once the tank is in service
New concrete tanks typically need a curing and flushing period before use, since fresh concrete can raise the pH of stored water
Installation is generally the most involved and highest-cost of the three materials for an equivalent capacity, which is why concrete tends to be reserved for large-volume or permanent in-ground applications rather than standard residential storage.

For most residential and light-commercial situations, steel and plastic remain the more practical choices on cost, lead time, and flexibility. Concrete comes into its own for large-capacity, permanent in-ground storage, such as commercial stormwater detention, where the scale and site-specific engineering are already a given.

Steel, plastic or concrete? Which water tank is right for your site

For most urban properties, particularly where the tank will be visible, positioned close to a dwelling, or specified as part of a building consent, steel is generally the better long-term choice. For basic rural storage where price is the main consideration and the tank will be out of sight, plastic remains a worthwhile option. For large-capacity or permanent in-ground storage, concrete is worth the extra cost. For everything in between, our buying guide walks through all four decisions that shape the right choice.

Where to go from here

If steel sounds like the right fit, browse a selection of our above-ground corrugated steel water tanks. If you need to store water beneath your driveway or lawn, explore our innovative Li-Lo flat plastic water tanks. Or talk to our team about your specific needs and requirements to get the best solution for your space.