Analysts want the Defence Department's financial reporting be more transparent and straightforward.

A parliamentary inquiry has recommended opening the books on Defence sustainment spending – the cost of keeping military equipment up to standard and ready for deployment.

Defence spending on the sustainment program will come in at $9.4 billion for 2017-2018, but it is expected to increase by 21.5 per cent over the next four years.

The Joint Committee of Public Accounts and Audit, led by Liberal Senator Dean Smith, found “variations in descriptive information, including no explanation of variations in full year outcomes, the use of different terminology across documents, and missing links between what was planned and what was actually achieved”.

The review found Defence's corporate plan, portfolio budget statements, additional estimates statements and annual reports should be changed to give a “clear read” of financial information and performance assessment.

“Defence's internal reporting regime should be open to the minister and the secretary, and report using plain language,” the report said.

“Terms like ‘overachievement’ should not be used to describe overspending. More broadly, the culture in the Department of Defence will need to continue to change to reinforce reform across this area.”

Defence told the inquiry that transparency must always be weighed against national security.

“Of utmost importance is the need to be cognisant of the national security risks associated with providing a high level of public visibility regarding military capability,” the department’s submission said.

The committee acknowledged the need to protect national security, but said “this should not preclude adequate parliamentary scrutiny”.

The inquiry said sustainment spending information should be consolidated to allow “for clear and easy scrutiny of sustainment expenditure”.

Currently, the data is spread across corporate plans, portfolio budget statements, additional estimates statements and annual reports.