It can cost more because the real numbers are starting to surface. The pre-election numbers were based on published data which was wrong. Right now the predictable capital expenses of this project are growing at rate that is out of control. There are also some major labour shortages as there are only so many people who can put the fibre in the ground -- mostly due to very old rules about certifications needed to work on anything involving electricity or working in a telco pit.
The first step of the new fibre network is to make sure that each exchange has a reasonable amount of fiber connecting it to the core. This hasn't been done yet and it will take years to do just that at the current rate. Once all the exchanges are hooked up, that breaks the ADSL monopoly and allows existing ISPs to install their own equipment in exchanges and drop their prices while offering naked DSL or port binding or even last mile 10 gigabit ethernet.
The second step is to roll out to the existing RIMs. These are remote extensions of the phone exchange switch and sometimes have ADSL at slow speeds. These tend to have fiber connections to them already but that fibre can't be upgraded to higher speeds since not all fiber is equal and the new stuff is more equal than others. The RIMs are part of the Node infrastructure and many already have ADSL2+ DSALMs but not enough back haul capability.
The third step is to upgrade DSLAMs combined with a rollout of replacement fiber where the existing copper is failing. The current list of areas at high priority for replacement will require every crew that is currently doing the core fibre install several years.
There are some areas that have two HFC networks, ADSL2+, 4G and are passed by competing fibre networks as well. Those areas are now last on the priority list.
Another issue that has annoyed many people is that the old maps of "when do I get fibre" would mark huge areas in "build out within two years" when the only parts that were planned was often connecting a new building or subdivision to the NBN. Those areas have all been removed from the map now.
When talking about the last mile CAPEX, the previous plan assumed nearly every house would be connected and factored in price increases that were above the current rate of inflation. Existing line cost about $36 per month which covers its written down CAPEX, the dial tone and minimal maintenance. Replacing that was expected to cost about $5,000 or now $7,000 per house at today's costs. While that can be factored over whatever term the government is willing to provide the loans, at the current rate it adds $35 per month to everyone's phone lines for 30 years which doubles the costs of the non-data user's phone. If there is any decrease in take-up, those costs start to raise rapidly. When the costs of 4G is less than the cost of a wired connection, what will the take up rate be in 5 years? If it isn't close to 90%, the finance plan breaks. With the demise of the wired phone and desktop computer combined with decreasing costs of wireless service that works anywhere, I can't see how the number of fixed wired services will not decrease.