The Ministry of Foreign Affairs says it has no specific concerns about the wellbeing of New Zealanders in Boston.
About 45 New Zealanders competed in the race, which ended when two bombs detonated within moments of each other.
The Ministry says it has had no reports of New Zealand runners or spectators being injured. It says its embassy in Washington is closely monitoring the situation and liaising with local authorities to determine if any New Zealanders need help.
People with family or friends in Boston should in the first instance attempt to make contact with them directly.
The ministry says if they require further assistance, they should call its consular division on 04 439 8000 and ask to speak to a consular adviser.
Two New Zealanders who ran in the marathon have described the experience.
Wellington business executive Andrew Wharton had finished his race and was back in his hotel room when the explosions happened. Shops were damaged and there was debris everywhere.
Former Alliance MP Laila Harre finished the marathon only five minutes before the blasts.
She told Nine to Noon there was no hysteria and the scene was well-controlled after the blasts.
Ms Harre said her instinct was to leave the area as quickly as possible and her husband, who was on a train, was evacuated.
Situation "serious and frightening" - John Key
The Prime Minister said the situation in Boston was serious and frightening, and the Government's thoughts were with those who had lost loved ones or had been injured.
John Key has been advised that 23 of the 45 New Zealanders registered to run the marathon were based overseas. He said the consular general in Boston had been working with two New Zealanders who had been near one of the explosions but they had not been injured.
