If the MGVWR of your truck and trailer when added up exceed 4500KG's then you need a sticker on both truck and trailer. Read the manufacturers sticker on truck and trailer to determine.
It's based on three things:
1. Actual weight (if both truck and trailer weigh more than 4500 kg's you need stickers)
2. Registered gross weight (if over 4500 kg's you need stickers and this would be indicated on ownership of power unit, it's what you pay for when getting val tag)
3. MGVWR (if manufacturer stickers of BOTH truck and trailer when added together exceed 4500 kg's you need stickers on both)