Mr Clark said he was disappointed others were using celebrities to influence people's decisions - a reference to Change the Flag's video featuring figures such as Dan Carter, Oscar Kightley and former Prime Ministers Jenny Shipley and Jim Bolger.
Mr Clark said the RSA did not have the resources to mount a major campaign - a Givealittle page it set up last year had received pledges of just $800.
"There's a lot of money out there at the moment being concentrated on encouraging people to change the flag and again I suppose it's the right of those people who do have those resources but we've got to be able to look our members in the eyes and say that what we spent, we spent responsibly. Our campaign is not about trying to change people's minds, as opposed to some of the other campaigns. But no sour grapes."
Lewis Holden, who leads Change the Flag, said the group had spent about $30,000 on its campaign so far, mainly on videos featuring high-profile New Zealanders who supported change. Further videos were planned.
But the group did not have the money for billboards or other grand gestures, he said. It had received donations, but Mr Holden would not name any of the donors: "We are not getting any money from political parties."
There is no limit on what campaigning groups can spend and no requirement to lodge a donations or spending return. One of the few rules for campaigners is for advertising to carry promoter statements.
Mr Holden said the group's focus was the first week of the referendum. "That's when we think most people will make up their minds."
Mr Clark said the most heartening aspect of the referendum was the amount of flag-waving it had promoted. He said governments had not done enough to encourage Kiwis to be proud of the flag.
"Take America for example, they're so proud of their flag, you see it everywhere and they raise it before school as we used to do years ago and sing the national anthem, as we used to do."
In the 1980s, the then National Government tried to enforce an old law which made it compulsory to fly the flag at schools. That was met with protests, including people chopping down flagpoles. The new Labour Government quickly repealed the law in 1984.