Sydney health authorities are on high alert after two people were diagnosed with measles after visiting multiple public spaces in the city's north and south.

Melbourne is in the grip of an outbreak of the highly infectious condition, with 11 cases recorded in the past fortnight after infectious people visited a number of crowded areas including the MCG, central business district and Melbourne Airport.

The Northern Sydney Local Health District on Tuesday issued an alert after diagnosing a patient who had returned from Melbourne.

The patient, while infectious, spent time in Warringah, Brookvale and Parramatta late last month.