"Probable cause" is a term better known to American television than to policing in NSW.
In NSW, the law talks about "reasonable grounds to suspect". Suspicion is a rather lower measure than "belief", so it doesn't take much to raise a suspicion.
The Stop, Search, and Seize powers are (mostly) described in the
Law Enforcement (Powers and Responsibilities) Act 2002.
One take-away thought...
Police seem to have this mysterious super-power to smell mull on the clothes, or in the cars, of young blokes, and their mates, who get smart-mouthy with the police.