[bjeanes]

It's not illegal for a company to offer an alternative to the NBN. I think it is however mandatory for a provider who can service a dwelling with NBN to upgrade from a legacy technology (i.e. DSL) to the NBN.

This does not preclude other high-speed internet options servicing those dwellings.

For example, https://www.lightningbroadband.com.au offers or plans to offer 100/100 (actually, some pages have mentioned 200/200 but maybe that's coming later or business only). A salesperson explicitly told me they plan to service NBN-connected areas too but that they'll probably prioritise other non-NBN metro areas first, because the demand will be stronger.