Sydney Airport is refusing to reveal the exact nature of a "technical issue" that left thousands of passengers stranded on Friday morning.

The Herald understands the issue that effectively shut down both the T1 International and T2 Domestic terminals until almost 7am was identified "well before" check-ins opened ahead of the first flights out from 6am.

Passengers at the domestic terminal wait to go through security after technical issues effectively shut down the airport on Friday morning. Credit:Nick Moir

Sydney Airport could not reveal the exact nature of the "technical issue" on Friday afternoon, except to say that it was not a power outage or "security issue" but a system problem that temporarily rendered CCTV useless, and interrupted passenger processing, including security screening.

A spokesperson for the airport told the Herald "essentially as soon as one of those aspects is not functioning, we can’t put passengers through security".