A new report has found that Australia's decade-long assistance mission to Solomon Islands achieved some results but at a 'massive and disproportionate' cost to Australia.

The Lowy Institute's report on the country's Regional Assistance Mission to Solomon Islands (RAMSI) says the whole operation cost Australia $A2.6 billion.

Launched on Thursday, its report is the first to put a price tag on the whole operation.

The report's author and director of the Myer Melanesia Program, Jenny Hayward Jones, says the costs built up largely because there was no clearly defined exit strategy at the beginning of the mission.

"(This) could have enabled the mission to draw down after some early successes," she told Radio Australia's Pacific Beat program.

"There was also a bit of a sense of 'mission drift' as the mission took on more and more tasks the longer it stayed."

Ms Hayward Jones says that before she started her research, there was no breakdown of expenditure across RAMSI's three main areas - law and justice, economic governance and the machinery of government.

"The breakdown that I publish in my paper is something that I asked for from the government," she said.

"This in itself is problematic because it shows the Australian Government was probably not doing the best job it could have of measuring its performance over the decade."

The research shows that the bulk of funds were spent on law and justice, which consumed just over $A2.1 billion or 83 per cent of the total cost.

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Australia's Regional Assistance Mission to Solomon Islands cost $2.6 billion: report