Victorian Premier Daniel Andrews has declared a state of emergency for four weeks as the state government steps up its attempt to slow the spread of coronavirus.

This declaration gives authorities powers under public health and wellbeing laws to enforce a ban on gatherings of more than 500 people and to fine or even detain people who flout quarantine orders.

The laws, which have never been used before, give the Chief Health Officer broad powers to do what he believes is necessary to contain the spread of the virus and reduce the risk to the public.

Fines of up to $20,000 could be imposed on anyone refusing to comply with a public health order issued under the legislation, and Mr Andrews warned on Monday morning there could be spot checks by police on people in 14-day self isolation after returning from overseas.