Three people are seriously ill in a Sydney hospital after eating death cap mushrooms - named for their poison that can trigger liver failure - at a New Year's Eve party in Canberra.

A 52-year-old woman and a 38-year-old man were flown to Royal Prince Alfred Hospital in a critical condition from Canberra's Calvary Hospital yesterday afternoon, a Calvary spokesman said.

Death cap mushrooms look similar to these straw mushrooms.

A 51-year-old man was taken to RPA in a critical condition from Manly Hospital, a spokeswoman for Northern Sydney Health Service said.

An ACT Health spokeswoman said a fourth person was believed to be in Canberra Hospital.