"It's shameful, it's absolutely shameful. I know there's quite a lot going on, with the flag being controversial at this time, but there's no reason to take a flag that belongs to other people and that belongs to the community."
The flag, which is also flown on Anzac Day, had initially been flown at half mast for the anniversary but was fully raised before it was stolen.
It appeared to have been untied and unclipped, with the thieves also making off with the halyard (rope) used to raise the flag, Mrs Dewes said.
She was unsure who might have taken the flag and, despite appeals on Facebook, it had not been returned over the weekend.
"There's no sign of it coming back, it's pretty rotten really.
"It's just the whole principle. We are a charity, so we don't have a lot of money to go buying another flag. It's over $1000 of equipment."
Many people had been angered by the theft and were hoping it would be returned, she said. The flagpole was added to the monument in 2012, with Mrs Dewes and her husband crafting it from an old steel power pole they found on their property.
They employed the skills of Eketahuna engineer Barry Burling, who donated materials needed to build the base, as well as his expertise and labour.
The theft has been reported to police.
Anyone with information should contact police or get in touch with the Friends of Anzac Bridge group on Facebook.