NSPCA spokesperson Celeste Houseman said inspectors were in the Graaff-Reinet area to monitor the culling.
She said 5 000 to 6 000 ostriches had been culled in late October and November in the Grahamstown area, while about 20 000 birds were culled in the Somerset East/Middelburg area where the avian flu outbreak was first detected in September.
Houseman said the disease was not spreading, but positive tests were cropping up as authorities worked through a backlog of testing.
"The government is again congratulated on taking swift and decisive action in terms of the outbreak of avian flu - but especially for including the NSPCA, the animal welfare organisation, in the process and for being transparent throughout," said Houseman in a statement.
Affected farms 'somehow linked'
Meanwhile, department of agriculture spokesperson Segoati Mahlangu said the current culling pointed to the effectiveness of the government's surveillance and monitoring programme.
He said the countrywide monitoring and "forward and backward tracing" of the movements of ostriches showed that affected farms in the various towns were somehow linked.
Mahlangu said the department had restocked farms in the Somerset East area with ostriches from outside the district on a trial basis.
"We are checking them on a daily basis... (and) will conduct three tests over an interval of 21 days, taking us to sometime in January, before we will be able to say if the area is clear of the virus."
Mahlangu emphasised the current culling was not a result of the disease spreading or a new outbreak.
"There is no indication of the disease anywhere else in the country. We have managed to contain the disease."
He said three farms have been quarantined in the Graaff-Reinet area, with about 2 200 ostriches to be culled.