The Lake Simcoe Region Conservation Authority (LSRCA) has issued a flood outlook for the watershed, which includes Newmarket, through to Thursday.
A flood outlook is an "early notice of the potential for flooding based on weather forecasts."
LSRCA said the warning comes as warmer weather and rainfall on Tuesday into Wednesday could cause flooding in low-lying areas. There is also risk of ice breakup leading to increased flow and ice jams in the rivers and waterways, including the Holland River and Fairy Lake.
"While widespread flooding is not expected, the possibility for flooding in various low-lying areas of the watershed exists due to the expected rainfall and melting of the snowpack," according to the flood statement.
"Watercourses will be flowing higher than normal causing local streams and rivers to become dangerous, especially in the vicinity of culverts and bridges. Children should be warned to stay away from all watercourses."
The Town of Newmarket is also reminding residents to stay away from bodies of water and not go on any ice on waterways and ponds.
"Be careful and stay off of local bodies of water such as Fairy Lake, the Holland River and stormwater ponds. Fluctuating temperatures make the thickness of the ice unpredictable and unsafe for skating and playing in these areas. Don't risk it," the town posted in a tweet.