Prime Minister John Key met Putin in Bali last month and said the Russian leader "gave me an assurance it [the case] would flow through the appropriate Russian judicial system" and that "in the end people need to consider their actions - the risks or otherwise - and I'm sure those protesters thought about all those issues".
Cameron told BBC Radio "they are not hooligans, they are protesters" but said nations needed "tough rules" to protect oil platforms.
He added: "Their charges have been dropped from piracy to hooliganism, but I still think that is excessive. They are not hooligans, they are protesters.
"I totally understand that countries have to have some quite tough rules to stop people invading oil platforms, but I have appealed to Vladimir Putin to try to de-escalate this and make sure that these people can go home."
His intervention came just hours before Russia said that the activists would now face the additional charge of resisting law officers - which carries a maximum sentence of five years.
Cameron's spokesman said Putin "understands" the Prime Minister's concerns and that the pair had agreed to stay in touch.
"This is now in the hands of the judicial authorities in Russia," the spokesman said. "We have made the representations that we have and we are going to use our diplomatic influence to continue ensuring that the Russian authorities understand the importance that we attach to resolving this."