My 2001 Volkswagen Polo's water bottle is full of a milky substance

Firstly, check that it's the coolant overflow bottle you're referring to and not the windscreen washer bottle; it sounds basic, but they can look similar from above.

If there is a milky looking fluid and it is in the radiator overflow bottle, then you potentially have a head gasket problem on your hands. The first thing to do is take the car to a workshop which can confirm or rule out the gasket drama, and take it from there. Most workshops will probably conduct what's called a TK test which checks the radiator's coolant for traces of the chemicals that are created during the engine's combustion process. In a normal engine, these chemicals can't reach the coolant, but if the head gasket has failed, the combustion and cooling systems can intermingle. When this happens, a murky, milky coolant is often the result.

The recent cold, wet weather across much of Australia has meant that many car owners are suddenly finding milky deposits under their engines' oil filler caps, too. This is also a classic symptom of a blown head gasket, but it can also be simple a build up of condensation in the engine thanks to the prevailing weather conditions. This is especially true if the vehicle is used only for short trips. A decent run at highway speeds will often be enough to get the engine hot enough to evaporate these harmless deposits.

But don't assume that this is the case, as a car that really does have a failed head gasket can easily overheat in such conditions and that can lead to complete engine failure. Have the car checked by a professional and you'll know how to proceed.