Some local Councils including those in S/W Sydney have implemented procedures to educate, warn, and where necessary fine people whose cultural or behavioural predisposition leads them to think that spitting is acceptable, and further, spitting can I believe be considered as common assault IF it’s directed at someone or at their property, done in an aggressive manner, or with the intention to intimidate and/or with no regard for the fact that a person on the receiving end would be likely to feel intimidated.
Regrettably none of that applies to your situation, and whilst the “peaceful enjoyment” provisions of the NSW Strata Schemes Management Act (1996) do (apply), those relate to occurrences within each Strata Plan and not acts within one that affects the residents of another.
It’s a difficult situation I agree, and the only practicable solution that I can think of would be to write to the Executive Committee Secretary of the adjoining Plan explaining both the situation and the physical location of the offender’s Unit and put that in the Owners Corporation’s letterbox, and perhaps try to find out the details of the Strata Manager for that adjoining Plan and copy your letter to them – they often have their details posted on front façade of the building or near its main entry door.