The company builds patrol vessels and frigates for the Australian Navy and has designed and built more than 300 vessels for more than 100 operators in 54 countries.
The breach was limited to the company's Australian operations, the company said, adding that it had referred the matter to the Australian Cyber Security Centre and the Australian Federal Police.
It said the attacker had threatened to sell some of the stolen data online, and sought a ransom to avoid doing so. Austal said it would not respond to such extortion attempts.
It indicated that ship design drawings, which were distributed to customers and fabrication sub-contractors, had been stolen, but said such material was neither sensitive nor classified.
"No company wants to lose control of its information, but there is no evidence to date to suggest that information affecting national security or the commercial operations of the company have been stolen," the company said.
The email addresses and mobile numbers of some staff had been accessed and these individuals had been informed, Austal said.
The Australian Defence Department said in a statement that no compromise of classified or sensitive information or technology had been identified thus far.
"This incident reinforces the serious nature of the cyber security threat faced by defence industry, and the need for industry partners to put in place, and maintain, strong cyber defences," the department added.
"Defence and the ACSC have provided cyber security assistance to Austal and are working with Austal to assess and mitigate harm."
Commenting on the incident, Daniel Lai, the chief executive of security firm archTIS, said: "The event highlights that both organised crime and state actors are targeting high value targets throughout their supply chain.
"If this can happen to Austal, a company that has significant resources to protect itself from cyber attack, then what about its sub-contractors and suppliers."
He said there was a need for a more co-ordinated response from industry, defence and government.