Whittlesea Crime Investigation Unit detectives are investigating a grass fire in Mernda on Wednesday.

The fire encroached on residential paddocks on the western side of Everard Road about 4.30pm.

The exact cause of the fire is yet to be determined.

Police wish to speak with three people, pictured, who were seen riding dirt bikes in the area prior to the fire.

Police urge anyone with information or who witnessed the incident to contact Crime Stoppers on 1800 333 000 or submit a confidential report online at www.crimestoppersvic.com.au.