NZR community rugby general manager Steve Lancaster said the postponement came after consultation with provincial unions and the Players' Association in the interests of wider public safety.
"Although it could be possible to play competitions and matches outside of Auckland and Coromandel if the Covid alert levels fell to level 2 before Saturday, with level 4 restrictions not being lifted until midnight Friday at the earliest, we believe the responsible decision is to make an early call and keep people away from our fields and venues this weekend.
"We understand this decision will be disappointing for some of our stakeholders but are confident the rugby community will once again act in unity to play our part in keeping Aotearoa safe."
Lancaster said NZR would review the status of all competitions beyond this weekend after the Government's next Covid update on Friday.
"Like all New Zealanders we will be guided by the Government and health authorities over the next seven days."
Whanganui had been scheduled to host neighbours Horowhenua-Kapiti at Cooks Gardens to defend the Bruce Steel Memorial Cup in their opening Heartland game.
Currently, they will now instead start against rivals South Canterbury at Timaru's Alpine Energy Stadium next Saturday, August 28.
While the WRFU Junior Rugby season had finished last Saturday, a number of other important fixtures have been affected.
The annual derby game between the Whanganui Collegiate and Whanganui High School 1st XV's was due to be played on Wednesday, while Thursday was to be the final of the WRFU Girls 10-a-side competition between WHS and Cullinane College.
Whanganui Pasifika was due to play Manawatu Pasifika on Saturday, while Sunday was to be the Whanganui Under-18 representative trial.
The status of these games will be confirmed at a later time.