Life imprisonment sentences are for an indeterminate period as long as the natural life of the prisoner. In NSW, there does not appear to be a maximum limit and a non-parole period cannot be imposed: Judicial Commission of NSW on Murder. In fact, five people in NSW have received natural life sentences. Further, under common law, a life sentence means a natural life sentence.
In other States or Territories in Australia, a minimum non-parole period may be imposed. However, there is no definite period of incarceration (NT for example has a standard non-parole period of 20 years but the judge can impose a non-parole period between 10-25).
It is a "life sentence" because there is no maximum imprisonment period. There is also no maximum non-parole period. In some jurisdictions, legislation allows a person initially sentenced to life imprisonment to be considered for parole after a certain period (i.e. there is a minimum non-parole period). This is solely at the discretion of the courts. The prisoner does not have a right to be considered for parole. There may not also be a parole period, meaning that the accused may be "on parole" for a rest of their natural life.
In other states such as in NSW, a life imprisonment means for the natural life of the prisoner and they will not be considered for parole. In other words, there is no minimum non-parole period.
In NT, for instance, a prisoner sentenced to life imprisonment can apply for parole after 20 years. In other States or Territories, the average minimum non-parole period is 10-25 years. This means, a prisoner sentenced to life imprisonment cannot apply for parole until that statutory period is up. However, even if the statutory period is up, there is no guarantee parole will be considered and/or granted.
In NSW, there is no parole period and so prisoners stay in prison for the rest of their natural life. This is also the case at common law.