That would be up to your negotiation skill and perseverance. You need to talk directly to Optus (I suggest Facebook or LIVECHAT). Different sales staff can give different answers so you could try a few times if knocked back.
The great thing about the NBN is it finally offers you freedom to find a better deal if Optus can't. Before now if you wanted cable speeds you had only two options. Now you can pick any deal buy any provider. Also switching to another NBN provider is pretty instant and easy. So put in your offer and see how you go.
FWIW though I suspect $60 is possibly a hard ask, IME Optus staff do have more wiggle room to apply a discount on a second service somewhere (so they could keep the internet at $70 but drop your mobile bill by $10).