Generally, the Council would be the relevant party in proceedings because it's their land. But the plaintiff would have a hard time laying responsibility on Council anyway, due to some common law immunities for government authorities responsible for roads and footpaths (unless they'd specifically been made aware of the risk).Even though if I was asked to sweep the leaves? What happens if somebody slips on wet leaves and breaks a leg? Still council's problem or mine?
I'd say that the only reason Council send the letter is so that if somebody falls, they can stand up in Court and say "we sent a letter asking people to remove the wet leaves", and that would probably be enough to protect Council from liability.
I seriously doubt they care whether you do or not.
http://elringtons.com.au/2011/08/who-is-responsible-for-my-injuries/