Assuming they will be replacing sleepers the same way as they have on the standard gauge to date, they will be using excavators fitted with a special blade head that allows the removal and insertion of new sleepers.
These machines were used on the Geelong line to replace sleepers of a nighttime, leaving the track usable the next day.
I think this one just digs the ballast out for insertion of new sleepers:
While this one looks like it can grab sleepers and shove them in:
These ones are stuck on the tracks - a sleeper remover:
And one for inserting sleepers: