"His health is not that good as everyone knows, we do this to give our well wishes," said Maneeka Campbell, founder of Thai Culture NZ and organiser of Saturday's event.
"People come tonight to pass on energy to the king, so that he can live the longest with us."
The festivities in New Zealand serve as a reminder to Thais living here of their relationship to a king who is regarded as the "father of our nation", Campbell said.
New Zealand Thai Society president Songvut Manoonpong said the relationship between Thais and the king was one of "deep devotion".
"For most of us, it is a genuine affection for a true leader who has helped the country solve many of its problems," he said.
Thais adore their royals, and many national holidays revolve around them -- the king's birthday is Father's Day and Queen Sirikit's is Mother's Day.
Images of the king and queen are at the centre of Thai restaurants, centres and businesses around the world, including here in New Zealand.
In a nation bitterly divided along political lines, Bhumibol is largely seen as a unifying force.
Manoonpong said despite anxiety over his health, plans are well under way for festivities here because many believed the "positive energy" would be transferred to the royals.
The queen's birthday will be celebrated at the Thai temple in Kelston on August 14 and again on August 20 at the Potters Park Events Centre.
The royal couple last visited New Zealand in 1962, where they were flocked by crowds during their eight day tour.
In Thailand, booths are set up for people to send their birthday wishes to the queen.
However many have instead been using them as a way to send get-well greetings to Bhumibol.
Images of the royal couple dominate public spaces there, from billboards to portraits and posters on high rise buildings, temples and market squares.
At the Thakilen railway station, American tourist Alison Beazley wrote: "No other man I know is loved by his people more than you, get well and long may you live your royal highness."
