There is no doubt that they can still do underground mining but...

There is no doubt that they can still do underground mining but it does have a potential impact as outlined in the 2017 annual report page 10:

Initial studies indicate the Pumped Hydro facility can be developed and operate concurrently with underground mining. However, the value of the underground mining would reduce due to ore left in the larger crown pillar required and increased mine development costs. Further studies will be carried out to optimise the combination of projects that returns the best value to shareholders.