He will arrive as Samoans brace for what they fear could be a dramatically rising death toll.
Last night, across the Pacific it stood at 189, TVNZ reported -- 149 in Samoa, 31 in American Samoa and nine in Tonga.
New Zealand deaths had increased to three, with another missing, presumed dead.
Diplomatic staff said there were grave fears for two others who were known to have been staying at the Taufua Resort in Lalomanu where huge waves created deadly havoc.
Bodies were still being found on the wrecked coastline and survivors said they had seen truckloads arriving in Apia.
About 20 New Zealanders are known to have been injured, and Acting Prime Minister Bill English said yesterday 10 were serious cases.
The air force last night evacuated 14 of them, with six family members. Three Britons and two Germans, all injured, were also understood to be on the flight which arrived at Whenuapai early this morning.
Diplomatic staff are still trying to contact about 200 New Zealanders thought to be in Samoa on the day of the quake. Some of them may have returned home.
Mr Key was due to arrive in Auckland from the United States about 7.30am, catching a commercial flight to Apia 30 minutes later.
He expected to arrive about noon and spend about seven hours in the Samoan capital before flying home on an air force Hercules.
Mr Key said he was making a brief visit because he didn't want it to distract from the life-saving work that was going on.
"It is important that I reassure Samoan leaders, face to face, that New Zealand stands ready to assist -- not just with short-term disaster relief but with long-term reconstruction."
Foreign Minister Murray McCully arrived in Apia yesterday to help co-ordinate New Zealand's emergency relief effort.
Military transport aircraft and air force Boeing 757s have been flying shuttle operations to take desperately needed supplies to Samoa.
