Western Australia has reported no new locally acquired coronavirus cases with more than two million people in the state halfway through a snap three-day lockdown.

The lockdown was ordered after a traveller likely became infected while in quarantine in a hotel and unknowingly passed it on to two other people in the community, according to authorities.

Testing of hundreds of people was still underway and state premier Premier Mark McGowan said it was too early to predict what the government would decide on Tuesday when the lockdown was due to end.