Kitchen and bathroom drains block for different reasons, and the fix is different too. Knowing which one you have changes what you should do.
What blocks kitchen drains
Cooking fat and oil is the number one cause. Even small amounts down the sink build up over months. The fat coats the inside of the pipe, then catches food scraps, hair, and detergent residue.
Food scraps caught in the drain trap. Particularly rice, pasta, coffee grounds, and anything that swells when wet.
Grease from washing greasy dishes. Soap and detergent can dissolve immediate grease but the residue still coats the pipe.
What blocks bathroom drains
Hair caught on the drain grate, then bound together with soap scum. The classic slow-draining shower.
Toothpaste, hair products, and personal care product residue. These accumulate in basin drains over years.
Wet wipes, even flushable ones. They do not break down like toilet paper. Toilets clog from these regularly.
Toys and foreign objects in homes with young kids. Particularly toilets and bath drains.
DIY fixes for kitchen drains
Hot water and dish soap: pour a kettle of boiling water down the drain, followed by a generous squirt of dish soap, followed by another kettle of boiling water. Repeat once or twice. This dissolves the grease coating in mild cases.
Vinegar and bicarb: pour half a cup of bicarb soda down the drain, then a cup of white vinegar. Cover the drain to keep the reaction inside. Wait 30 minutes, then flush with hot water. Works for mild buildup.
Avoid chemical drain cleaners. They are aggressive on pipes and rarely work on grease-based blockages.
DIY fixes for bathroom drains
Pull the drain grate and pull out visible hair with a hooked bit of wire or a drain claw tool (a few dollars at Bunnings).
For shower drains, a plunger works if you cover the overflow first.
For basins, the trap under the sink can be unscrewed and cleaned out by hand. Put a bucket underneath first.
When to call a plumber
Kitchen blockages that DIY does not clear in one go are usually deeper in the line. Plumber needed.
Bathroom blockages caught early are usually DIY-fixable. If you have not cleaned the drain grate in over a year, do that first.
Any blockage affecting more than one fixture is a main line issue. Plumber needed.
Prevention
Kitchen: never pour cooking fat down the sink. Wipe pans with paper towel and bin it. Use a sink strainer for food scraps.
Bathroom: clean drain grates every couple of months. Replace any old or worn shower head, as some build up scale that flakes off and contributes to blockages.