Headlights on modern day vehicles are made from a type of thermoplastic polymer called polycarbonate, as opposed to older vehicles where they were made from glass.
Polycarbonate has huge advantages over glass in that it is easily moulded into complex shapes, is very impact resistant and very transparent to visible light, better then many types of glass. The disadvantages of this type of material is that it is not very suitable for long term UV exposure and can discolour and yellow. Given the extreme strength of UV in North QLD, bad headlights are a common sight and will take away the overall look of a vehicle, but more importantly will lower the amount of light your headlights transmit onto the road leading to dangerous night time driving.
The good news is this damage is easily reversible, even on the most damaged headlights but does require a proper multiple stage process of reversal and protection. Simply polishing the headlights with an abrasive substance will only give some improvement and the headlights will quickly return back the way they were in a matter of months or weeks due to no long term protection being applied.