Hostages in the Sydney cafe siege have reportedly been forced to call a number of media outlets in Australia.

Tom Steinfort, a journalist with broadcaster Channel Nine Australia, said it had been contacted by three hostages, including one who had been "forced by the gunman" to call its office.

The captor's demands had been passed to police and the hostages's relatives notified, he added.

2GB Radio said its presenter Ray Hadley had also been contacted by a hostage and they spoke off-air.

Producer Laura Parr said Mr Hadley could hear the gunman "giving instructions to (a) young hostage off-air".

There was also reports that one hostage inside the Lindt Cafe managed to send a tweet urging police to be "cautious".

Meanwhile, Ten News anchor Hugh Rimington said they received a video in which a hostage "relays the gunman's demands".

Several local media outlets said they were complying with a police request not to relay the offender's demands.

Chris Reason, a journalist with Seven News Sydney, said he could see the hostages from his newsroom in Martin Place.

He said: "From inside Martin Place newsroom, we've counted around 15 hostages - not 50. Mix of women, men, young, old - but no children."

Mr Reason said the hostages were "huddled at one end of Lindt Cafe" and the gunman was "rotating hostages, forcing them to stand against windows, sometimes two hours at a time".

On its Facebook page, Lindt Chocolate Cafe said: "We would like to thank everyone for their thoughts and kind support over the current situation at the Lindt Chocolate Cafe at Martin Place.

"We are deeply concerned over this serious incident and our thoughts and prayers are with the staff and customers involved and all their friends and families.

"The matter is being dealt with by the authorities and we are waiting for any updates from them."