Detective Superintendent Mark Hutchings, from the New South Wales Police, identified the tourists as Richard Cousins, 58, Emma Bowden, 48, Heather Bowden, 11, Edward Cousins, 23, and William Cousins, 25.
The pilot, Gareth Morgan, 44, also died in the crash.
Hutchings said Heather was with her father onboard and that all passengers knew each other. He added that police had already been in contact with UK authorities.
Police are working with the Australian Transport Safety Bureau to recover the wreckage of the plane, which is submerged in 13 metres of water near Cowan, north of Sydney.
An investigation into the cause of the fatal crash has begun, with a preliminary report expected within 30 days. However, authorities have warned that it may take up to a year to find out what happened.
The seaplane was part of the Sydney Seaplanes business, which has operated since 2005 with no previous record of mishap. Further seaplane flights have been cancelled until further notice.