UPDATED 7:15pm: The fire that gutted a million-dollar cafe in one of the country's most popular parks is being treated as suspicious.

Both the police and Fire Service are investigating this morning's blaze at the Cornwall Park Cafe in Auckland, and believe it was deliberately set.

Specialist fire investigator Russell Dickson told APNZ he has identified - but won't detail - an "introduced source of ignition" and sent away samples for testing.

The cafe was still under construction, and did not have electrics or fire alarms when the fire was started about 2:30am.