The first case of COVID-19 related to the plant was detected on Monday.

Of the 24 total new cases of COVID-19 detected in WA yesterday, nine were related to Kemerton.

It took the total Kemerton cluster to 16.

"They are all related to Kemerton and Albemarle in one way or another," WA premier Mark McGowan said yesterday.

"[The plant] has been under construction now for a few years and is an important investment for the state in terms of downstream processing of lithium."

McGowan said the large, 90 hectare site was divided into a number of different working areas.

"One contractor in particular has been affected by this outbreak," he said.

"There are 30 contractors on site but one contractor and one work area in particular have been affected."

The work area has been temporarily suspended, with all close contacts in isolation, some at a beachside resort north of Bunbury.

McGowan said 99 close and casual contacts had been identified, of which 73 had tested negative to date.

An Albemarle spokesman said all workers at Kemerton were vaccinated, in line with WA regulations, and the site continued to adhere to all COVID-19 health protocols.

"The site is working with WA Health COVID-19 contact tracing," he said.

"The wellbeing and safety of our workforce and communities continues to be our priority."

The Kemerton development, a 60:40 joint venture between Albemarle and MinRes, comprises two 25,000 tonne per annum lithium hydroxide modules.

The US$1 billion project has already been plagued by delays due to WA's tight labour market.

Kemerton I was due for construction completion by the end of 2021, but Kemerton II was twice-delayed last year until the June quarter.

First lithium hydroxide is expected by mid-year.