Hi @andyyu,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
The first thing to check is whether the new cartridge is fully seated. Even a small misalignment can allow cold water to pass through. Turn off the water, remove the handle and cartridge and make sure the rubber seals at the base are clean and sitting in the right spots. Then reinstall the cartridge in the exact orientation as the original.
If the cartridge is seated correctly, the next likely issue is worn internal O-rings or seals in the tap body rather than the cartridge itself. These rubber parts can harden, tear or flatten over time. When that happens, they may still seal against the hot side but not the higher-pressure cold side. Inspect the inside of the tap for any damaged or deteriorated rubber and replace anything that looks worn.
Finally, take a close look inside the tap body for corrosion, cracks or pitting. If the surface where the cartridge sits has been damaged over time, no new cartridge will seal correctly. A torch can help you see inside the housing. If the metal is uneven or corroded, replacing the tap body is usually the only lasting fix.
Run through these steps and let me know what you find. I am happy to assist further if required. If you require further assistance, could you upload some photos of your tap so we can get a better idea of what we're working with?
Let me know how you go.
Jacob
