Ms Simcock said the activists would remain in place "until we're really sure Mars has got the message".
They were also hanging a banner saying "Stop bad tuna" from the paws of the two billboard cats.
Activists hang a banner on the Mars Petcare building. Photo/ Stuart Munro Two other cats painted on the factory were holding Greenpeace signs.
Ms Simcock said they were making the cats into "cativists".
Greenpeace are campaigning against Thai Union globally.
Ms Simcock said there was an meeting between Mars and Thai Union in the United Kingdom at 8pm New Zealand time.
The action hopes to influence the meeting.
The Greenpeace truck was blocking access to the factory.
Police had been to the site but had not taken any action. A Police spokesperson they were aware of the protest and were monitoring it.