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July 13, 2026At first glance, a double bowl sink looks like two separate sinks sitting side by side, but underneath, they both drain through one connected plumbing system. That’s the tricky part people often overlook when installing a kitchen sink with two basins instead of one.
In this guide, we’ll break down what makes double kitchen sink plumbing different from a standard single sink setup, whether it’s realistic to tackle as a DIY project, and what the general installation process actually looks like from start to finish.
What Makes Double Sink Plumbing Different From a Single Sink?
Before you touch a single pipe, it helps to understand why a double bowl sink isn’t just ‘a single sink setup, twice.’
One Shared Trap, Not Two
Both bowls connect into a single trap through a connector fitting, not two separate traps. AS/NZS 3500.2 outlines a double bowl connector and trap as the correct way to join two bowls to one trap.
Running waste through two traps in series (often called double-trapping) is a non-compliant setup that leads to drainage issues, and it’s worth flagging clearly since a few other guides online present dual-trap setups as though they’re a legitimate option. They’re not.
The Extra Fittings a Double Sink Needs
A double sink brings a few more parts into play, including a connector (sanitary tee) joining both tailpieces, two tailpieces instead of one, and more compression joints tucked under the cabinet.
That means more potential leak points if anything isn’t fitted properly, including where you’ll eventually be replacing a P-trap down the track.
Why Venting Becomes More Important
With a shared trap and a longer pipe run between two bowls, proper venting matters even more than it does on a single sink. Get it wrong, and one bowl can gurgle or fill up while the other drains, something we’ll dig into later in the troubleshooting section.
Is Plumbing a Double Sink a DIY Job in Melbourne?
It depends on which part of the job you’re talking about. Some tasks are perfectly fine to handle yourself; others cross into territory that’s simply not legal to DIY.
What Homeowners Can Legally Do Themselves
Minor jobs like replacing a tap washer or clearing a blocked drain with a plunger are fine to do on your own. But the moment you’re altering drain connections, tailpieces, or the trap assembly itself, you’ve moved into drainage work, and that’s a different category altogether.
If you’re unsure where that line sits, it’s worth checking what’s legal to DIY in Australia before you start pulling pipes apart.
Why the Drainage Work Needs a Licensed Plumber
Under Victorian regulations, drainage work has to be carried out by a licensed plumber, who can then issue a compliance certificate for the job.
That certificate isn’t just paperwork. Without it, your insurance may not cover any damage that turns up later if something’s gone wrong.
On top of that, any fittings used need to be WaterMark-certified, which isn’t something you can guarantee buying random parts off the shelf.
How Double Kitchen Sink Plumbing Is Installed
While the compliance side means this isn’t a full DIY project, it still helps to know the general stages a plumber works through:
1. Check Clearance Under the Cabinet
Before anything gets connected, there needs to be enough room for two tailpieces, the connector fitting, and the trap to all sit together without cramming.
Cabinet width and drain outlet height both need to accommodate these extra fittings, otherwise things won’t line up properly once installed.
2. Connect Both Bowls to the Shared Trap
Tailpieces run from each bowl’s strainer down to the connector fitting, which then joins onto the P-trap. From there, it connects to the waste line at the correct fall, so wastewater actually flows the way it’s supposed to instead of sitting stagnant.
3. Fit the Water Supply Lines
Hot and cold isolation valves get connected to the mixer tap, and it’s worth using WaterMark-certified flexible hoses for these supply connections rather than cheaper alternatives that aren’t rated for the job.
4. Test the Finished Setup
Once everything’s connected, both bowls get run together to check for leaks at every joint.
It’s also the point where a plumber confirms neither bowl backs up or gurgles when the other one’s draining, which is your first real sign the venting’s been done correctly.
Common Double Sink Plumbing Problems
Even with a properly installed setup, double sinks tend to run into a couple of recurring issues worth knowing about.
One Bowl Gurgles or Fills When the Other Drains
This usually comes back to a venting issue, the same one flagged earlier in this article.
It can also be caused by a partial blockage sitting at the connector fitting or shared trap, which disrupts the flow enough to push waste (or air) back up into the other bowl.
Both Bowls Draining Slowly
Since the connector fitting sits between two sources of waste rather than one, it tends to collect grease and food buildup faster than a single sink’s plumbing would. Over time, this narrows the passage and slows drainage on both sides at once.
If you’re dealing with a kitchen sink not draining properly, it’s a good idea to check the connector fitting first, since that’s usually where double sinks run into trouble before anywhere else.
FAQ About Double Kitchen Sink Plumbing
Does a double sink need two traps?
No. A single trap fed by a connector fitting that joins both bowls is the standard, compliant setup. Running two traps in series (known as double-trapping) isn’t compliant, and it tends to cause drainage issues rather than solve them.
Why does one side of my sink gurgle when I drain the other?
This is usually a venting problem. When the system isn’t vented properly, air can’t enter the drain line the way it’s supposed to, so instead it gets pulled through the trap on the other bowl.
That’s what causes the gurgling sound, and in some cases, it can even siphon water out of that trap.
Can I connect a dishwasher to a double sink’s drain?
Yes. The dishwasher drain hose ties in through the connector fitting or via a high loop, and it doesn’t need a separate trap of its own. It’s a simple addition once the double sink’s drainage is already in place.
Do I need a licensed plumber to install a double kitchen sink in Melbourne?
For anything involving the drain connections, yes. This falls under regulated drainage work in Victoria, which means it needs to be carried out by a licensed plumber who can issue a compliance certificate once it’s done.
How much does it cost to have a double sink plumbed in?
Based on general Melbourne market research, having a new double sink installed, including the plumbing work, typically sits somewhere around $500 to $1,200.
The exact cost depends on things like the existing pipework and how easy the space is to access.
Conclusion
At the end of the day, a double kitchen sink isn’t two plumbing systems living side by side, it’s one shared trap and connector fitting doing the work for both bowls. Get that part right, along with the venting, and everything else tends to fall into place.
Getting the plumbing right on a double kitchen sink is actually easy once it’s done properly, but a poorly fitted connector or missing vent can mean gurgling drains and callbacks down the track.
That’s why, sometimes it’s better to call the professionals. Neighbourhood Plumbing installs and reworks kitchen sink plumbing across Melbourne to AS/NZS 3500 standards, with WaterMark-certified fittings and a compliance certificate for your records.
Get in touch with our kitchen and bathroom renovation plumbers or call 0488 885 122 for a personalised quote.
About the Author
From unclogging the toughest drains to repairing leaking toilets, Ricky is a seasoned plumber who has brought his craftmanship to homes across Melbourne. He is passionate about sharing plumbing tips just as much as he is about fixing your plumbing issues. So, when your taps or pipes begin leaking, you know who to call.
