Rhona Smith said the government must immediately release the detained opposition leaders and lift a ban on the opposition taking part in the July 29 general election.
The CNRP was dissolved and its lawmakers banned from politics in November after the Supreme Court ruled that it had tried to overthrow the government.
The Cambodian Senate has long been seen as a rubber stamp body with the balance of power falling with the ruling party and its cronies.
Sam Rainsy added that Japan had strong leverage over Cambodia due to its considerable development assistance and investment.
A ruling Cambodian People’s Party spokesman said Sam Rainsy’s appeal to voters was “in vain”, adding that turnout was expected at between 60 and 70 percent.
Prime Minister Hun Sen on Monday blessed the Cambodian people during a Khmer New Year ceremony, declaring his victory over the opposition party.
Cambodia National Rescue Party (CNRP) was dissolved by Supreme Court last November at request of PM Hun Sen's government, which alleged it was plotting to take power with the help of US.
Local nongovernmental groups also invited to monitor July vote ruling party is expected to win because main opposition party was dissolved.
International delegates are required by law to be invited to observe Cambodian elections.
The claims come more than two years after the government passed a controversial law regarding the conduct of non-governmental organizations in Cambodia.
Cambodia’s Military Prepares to Battle an Uncontested Election
The protesters were responding to threats made by Hun Sen to “chase and beat” Cambodian-Australians who burned his effigy during a visit to the country for the Asean-Australia Summit later this month.