The aircraft, operated by the carrier's regional arm QantasLink, were removed from service on following advice from the manufacturer, the airline said in a statement.
An overseas Q400 operator had experienced a similar problem, prompting Qantas to investigate the fleet, it said.
Cracks were discovered in a major component of a landing gear fitting in five of its 21 Bombardiers, with the remaining 16 unaffected and to remain in service.
Qantas chief executive Alan Joyce said the issue was not an immediate safety concern, but needed to be rectified before the aircraft could be returned to service.
A resolution is not expected until the end of next week, forcing the cancellation of up to seven QantasLink Sydney-Canberra flights each day over that period.
Affected customers will be contacted and put on the next available flights, Qantas said.
Supplementary flights will also be operating and larger aircraft will be used wherever possible to minimise inconvenience, it said.
"Our approach to this issue, which requires the replacement of a main landing gear fitting component, is consistent with Qantas' proactive, conservative and safety-first approach to every part of its operations," Mr Joyce said.
