Victorian Premier Daniel Andrews had revealed an additional 374 cases of COVID-19 have been recorded in the state.
He said three more people died yesterday, a woman in her 100s, a woman in her 90s and a woman in her 80s.
Mr Andrews said mandatory mask wearing comes into force from midnight Wednesday for Melbourne and Mitchell shire residents.
He said the measures put in place were having a direct impact.
Mr Andrews said there were still a small number of people who were not taking the situation seriously and were not doing the right thing.
"Please change the way you are behaving," he said. "The stakes are very high."
Watch Mr Andrews address the state below
Yesterday, he announced Victoria recorded 275 new cases and an aged care resident in her 80s died from the virus.
Premier Daniel Andrews said on Monday, it was too soon to know if the curve was flattening.
"We had a very big day Friday and then we had a substantial drop-off, even though we had done more tests," he said.
Mr Andrews said it could take up to two weeks for the effects of the stage three lockdown in Melbourne and Mitchell Shire to be reflected in the figures.
"Let's wait to see how the week unfolds, certainly, we're always happier to report lower numbers than higher ones, particularly when you see testing continue at such a high rate," he said, noting about 26,000 tests were conducted on Sunday.
- with AAP