Reports of problems at voting booths and allegations of ballot fraud have soured the mood among some in a country which has a history of electoral violence and corruption.
Polling in Papua New Guinea has been hampered by reports of disruptions and voters being left off the electoral roll, but the head of an international election observer group said on Sunday there was no evidence they were deliberate.
The two-week long election to decide who will lead the resource-rich South Pacific nation began on June 24, pitting 3,332 candidates from 44 political parties against each other for a place in the 111-seat parliament.
"There has clearly been problems ... but to be fair, in our observation, the government has endeavored to address these," Sir Anand Satyanand, chairman of the Commonwealth observer group, told Reuters by phone.
Problems with the 2017 electoral roll have meant that the 2012 roll was being used in some areas, preventing some people from casting votes.
Logistical issues, poor transportation links and bad weather had disrupted voting in other parts of the country.
Prime Minister Peter O'Neill, who is in a strong position to retain power, has rejected accusations by opposition politicians that he is to blame for the election issues.
"The electoral roll is the responsibility of the Electoral Commission and is independent," he said in a statement on Sunday.
"Failed leaders can make any claim they like, but they never back this up with proof."
Satyanand said he had seen no evidence to suggest the issues were planned.
Despite its mineral wealth, which includes Exxon Mobil's $20 billion LNG plant, most of Papua New Guinea's nearly eight million people live at subsistence level on islands, atolls and in remote mountain village.
The rugged terrain and poor infrastructure make holding an election difficult and expensive, according to Associate Professor Sinclair Dinnen, a Melanesia expert at the Australian National University.
"The larger background is that the funding to the Electoral Commission was cut back this time because of the fiscal crisis in PNG, and that too has led to problems," Dinnen told Reuters by phone.
PNG's credit rating was downgraded to B2 by Moody's in April last year, reflecting ongoing balance of payments pressures that have been compounded by slump in oil and gas prices.