South Australia has signed up to the federal government's mental health and suicide prevention plan, allowing for combined spending of $128 million to improve services across the state.

The SA government will provide $62 million and the commonwealth $66 million over five years to improve programs and support the establishment of four new mental health centres.

The SA Liberal government has also pledged a further $5.5 million to boost mental health services for children, should it be returned at this month's state election.

"This package of programs and services is designed to support and care for South Australians when they need it most so that we can build a mentally resilient and healthy community," Premier Steven Marshall said.

Under the agreement, a new Headspace site providing support to young people aged 12 to 25 years struggling with mental health issues will also be established.

The plan further recognises the need for more support for veterans and for the state's Indigenous communities.

But Labor said the state government had rushed to sign a deal with the federal government that other states, including Victoria, Western Australia, Queensland and Tasmania, had so far refused.

"It's unprecedented to sign an intergovernmental agreement during caretaker conventions and clearly this has been rushed to paper over the government's inaction on the mental health crisis," Labor Health Spokesman Chris Picton said.