But observers say that the warning is politically motivated and lacks legal basis.
Comfrel says several hundred members will observe general security but not monitor the July 29 vote.
A representative says a Chinese mining firm is responsible for contaminating the water.
The near-assassination comes weeks after a local official threatened him with death.
The main opposition group, banned in late 2017, has called for a boycott of what is effectively a one-party race.
The statement comes as two former RFA reporters mark six months in detention on espionage charges.
He says the CNRP will succeed despite a political ban because it enjoys the support of the people.
A judge maintains the 2015 sentences after two minutes of deliberation, but provides no reason.
Clean water distributed by authorities and NGOs may soon run out, they fear.
They say the purchase of The Phnom Penh Post by a Hun Sen-linked businessman strikes a new blow against press freedom.
Sam Rainsy urges Cambodians to boycott the general election in July as long as the Cambodian National Rescue Party cannot participate.
The latest threats from the government come amid a call for a boycott of the upcoming election.
Not everyone who died or fell ill could have been sickened by rice wine, they say.
The Phnom Penh Post’s new owner fires staff over article detailing his ties to Prime Minister Hun Sen.
Lawyers, daughter say the ailing CNRP leader's case is purely political and requires negotiations.
The warning comes as former opposition chief Sam Rainsy urges voters to avoid the July polls.
Hun Chea is arrested, tried, and sentenced in one day—a near record for Cambodia’s sluggish courts.