And Nona has also lost a bid to overturn convictions for molesting a girl, the older of two sisters, at her Canberra home in the mid-1990s.

Nona has been behind bars since 2012 for the crimes, which occurred on five separate occasions after he came to the ACT, a vastly different environment to his birthplace, the remote Badu Island in the Torres Strait.

He was found guilty of flashing the girl, pinning her down and molesting her on her bed, sexually assaulting her on a staircase, and raping her before an athletics carnival.

Another rape occurred when the girl was left alone in the house with Nona, after her sibling was taken to hospital.

Nona later established himself as a leading Indigenous artist, with works in top Australian and international galleries, and national institutions, including Parliament House.

He was sentenced to seven years and six months on nine charges early last year, with a non-parole period of three years and nine months.

But Nona appealed his sentence and conviction, prompting a hearing in the ACT Court of Appeal in February.

The prosecution counter-appealed, arguing the sentence was manifestly inadequate.

Both appeals were dismissed on Friday.

Nona, represented by barristers John Purnell and Shane Gill, had argued the verdicts handed down by the jury were unsafe and unsatisfactory.

That was mainly because of the girl's evidence about the dates of the offences, which they argued was flawed and cannot have been accurate.

They also argued there was limited opportunity for most of the offences to have occurred.

The girl's evidence about the hospital incident was also said to be inconsistent with hospital records, which potentially showed she went with her sibling that night.

But a full bench of the ACT Court of Appeal rejected the arguments against the verdicts, saying the jury's attention was expressly drawn to the hospital notes, and they were told to examine the complainant's evidence with "great care".

The judges - Chief Justice Helen Murrell, and Justices Richard Refshauge and Michael Wigney - said the jury members were also told to closely scrutinise the girl's evidence about dates.

"Inconsistencies in relation to dates do not necessarily undermine the critical aspects of the evidence of a young complainant who is asked to recall highly traumatic events many years later," the judges wrote.

Nona's appeal also argued there were four errors made in sentence.

He argued the judge should have found his physical abuse during childhood contributed to the deprivation of his adult development.

Nona also argued his deprivation should have been used to assess his culpability for the offences, and that a connection between his alcohol abuse and the crimes should have been considered.

The prisoner said his alcoholism and associated displacement from the Torres Strait, where he was born, to the ACT, where he felt alienated, should have been taken into account when determining his culpability.

But the appeal court found the sentencing judge took the issues of displacement, physical abuse, and alcoholism into account properly. The court also agreed with the sentencing judge in finding that there was no direct link between alcohol abuse, displacement and the offences, meaning they could not be used to reduce culpability.

"The appellant did not explain how, in the circumstances of this case, alcohol abuse or social alienation mitigated the objective seriousness of the appellant's repeated, forceful violations of the complainant," they wrote.

The court also rejected the prosecution's appeal for a tougher sentence.

In separate proceedings, Nona was also found guilty of raping and impregnating the younger of the sisters, and was sentenced in April to five years imprisonment, taking his total sentence to nine years with a non-parole period of four years and six months.