New Zealand is sending aid to disaster-struck Tonga, but ash on the capital's main airport runway is preventing relief planes from landing.
It will also take days before military ships with supplies will reach the islands, said New Zealand authorities.
It sparked a tsunami and severed an undersea cable, cutting off the country from the outside world.
New Zealand and Australia have done surveillance flights to assess the damage.
On Tuesday New Zealand foreign affairs minister Nanaia Mahuta said a C-130 Hercules aircraft was on standby to fly to Tonga to deliver humanitarian aid including collapsible water containers, generators and hygiene kits.
"However images show ashfall on the Nuku'alofa airport runway that must be cleared before (the plane) can land," she said.
Ms Mahuta added two navy ships carrying water supplies, disaster relief stores, and a rescue helicopter will be dispatched to Tonga. However the ships are expected to take three days to arrive.
"One of the greatest needs is to provide water purification and clean drinking water," Mr Matheou said.
A distress signal has been detected from two small, isolated Tongan islands, according to the UN.
But the Red Cross has said reports suggested the overall damage was not as bad as had been feared.
"We believe that from the information that we can put together that it is not as catastrophic in those major population centres as we first thought that it might be, so that's really good news," said Katie Greenwood, who is co-ordinating the organisation's response from Fiji.
Communications with the island chain remain extremely limited, making it difficult to establish the scale of the destruction.
Tongans living abroad have been anxiously waiting for news from relatives and loved ones back home, as reports say it may take up to two weeks to restore phone and internet lines.
The Red Cross said even satellite phones, used by many aid agencies, had poor service due to the effects of the ash cloud. The organisation estimates that up to 80,000 people may have been affected by the tsunami.
Some officials have voiced concerns over relief efforts resulting in a spread of Covid in the country, which only recorded its first case in October.
"We don't want to bring in another wave - a tsunami of Covid-19," Tonga's deputy head of mission in Australia, Curtis Tu'ihalangingie, told Reuters.
Tonga is made up of 170 islands scattered over about 700,000 sq km. About 100,000 people live in Tonga, the bulk of them on Tongatapu Island.
The underwater volcano erupted on Saturday, about 65km (40 miles) north of the capital Nuku'alofa.