The broadcaster said it was unable to air several shows on Sunday, including Weekend Today.
Nine said it was investigating whether the hack was "criminal sabotage or the work of a foreign nation".
Australia's Parliament was also investigating a possible cyber-attack in Canberra on Sunday.
Assistant Defence Minister Andrew Hastie said access to IT and emails at Parliament House had been cut as a precaution. He said this was done in response to issues affecting an "external provider", without elaborating.
