Phil Blackwood, 32, who managed the VGastro Bar in Yangon, was sentenced earlier this month to 2½ years in prison after he used a psychedelic image of Buddha wearing headphones to promote a cheap drinks night at the venue.

Another manager at the bar, Htut Ko Ko Lwin, and bar owner, Tun Thurein, were also sentenced to 2½ years jail with hard labour for insulting religion.

The case came amid a surge in Buddhist nationalism in Myanmar - which emerged in 2011 from half a century of military rule - with monks forming groups aimed at promoting the country's Buddhist character.

Blackwood's parents, Brian and Angela, told the ABC that they feared for their son's health due to the conditions in the notorious Insein Prison in Yangon where he was imprisoned.

"He sleeps on a wooden pallet, no bedding is allowed, no mattress, no pillow," Brian told the ABC from his home in Wellington.

"We did manage, after about 50 days, to get a blanket to him so that he could actually have something to lay on, and then later on we managed to get him another blanket so that he could fold it up and use it as a pillow.

"He has a hole in the ground for a toilet and he has a bucket of water that he washes in, keeps himself tidy with.

"I don't believe that we understand half of what he is suffering."

Blackwood's fiance, Noemi Almo, and their seven-month-old daughter can visit him in prison once a fortnight, Brian said.

"Obviously his health is the first issue that we're concerned about," Brian said.

"He does have a chest condition, he has asthma, which I don't think the heat over there will help."

Blackwood's mother, Angela, said her son's prison sentence was "devastating".

"Every time I go to bed, and I'm sleeping on a nice comfortable bed, I can see him sleeping on a wooden pallet," she said.

"We worry about the hygiene conditions, him getting infections, malaria. You know, the more you think about it, it just sort of goes on and on. There's always something else to worry about."

Following the sentencing on March 17, rights groups condemned the verdict as an assault on freedom of expression and called for the release of the three.

When the Buddha image was posted online it sparked an angry response, and was removed from the bar's Facebook page.

Blackwood said sorry online and repeated his apology in court. But the judge, Ye Lwin, said that although Blackwood apologised, he had "intentionally plotted to insult religious belief" when he uploaded the poster on Facebook.

Blackwood's parents have launched an online petition calling for their son to be freed.