A plant to bring in cheaper workers when WA public servants leave their jobs will save $450 million less than expected, analysis shows.

Treasurer Mike Nahan revealed the saving downgrade in a budget estimates hearing this week, in which he also conceded the Government was struggling to meet its target of 1,500 voluntary public sector redundancies.

Dr Nahan was quizzed about the savings in the Government’s workforce renewal policy, which replaces employees vacating positions with a new worker on 60 per cent of the original salary.

But the policy had exemptions for some frontline workers and departments, which Dr Nahan said led to a decline in expected savings.

From the current financial year through to 2017-18, the policy’s forecast savings have now declined by $458.2 million.

Dr Nahan also gave word on the Government's voluntary redundancy scheme, which is aiming for a target of 1,500.

He says there have been 1,339 actual separations, but it is getting tight with just a few weeks to go.

Dr Nahan was asked if the target would be met, he said “that is yet to be determined, we might be a bit below that”.

The Treasurer said 420 redundancies came from the Health Department, 272 from education, 200 in Training and Workforce Development, 100 from Agriculture and Food, and 96 from Corrective Services.