Sam Cataki, 36, who has lived in Neyland for the past five years, has also been told he can no longer work.
He lost his job at Milford Haven Port Authority on Wednesday and, with two young children aged one and five, said he fears for his family's future.
The Home Office has been contacted for comment.
Before he left the Army in 2013, Mr Cataki said he applied for citizenship in the UK but believes he was rejected because of three points on his driving licence.
"They say it doesn't count as 'good citizenship' which is one of the criteria," he explained.
He was given further leave to remain until his points cleared and told he would then be able to re-apply for citizenship.
"I applied again last year but that's when they told me it was void. They're telling me to re-apply for settlement [with] Armed Forces indefinite leave to remain but I'm still waiting," he said.
"I normally don't complain about anything but I have followed every procedure they've asked.
"After serving they should be giving more to ex-servicemen but it's like they have neglected and left me. They thank you for your service and then you're forgotten about."
Mr Cataki had been working in Milford Haven for two years and said the situation was also putting his wife Seini's visa status at risk.
"We really enjoy it here, we just want to work," he added.
"I've spent about Â£5,000 on applying for visas but without a job it will be difficult to afford applying again."
Stephen Crabb, MP for Preseli Pembrokeshire, has been assisting Mr Cataki since 2014.
In a message on Facebook on Friday, Mr Crabb said: "Lots of strong support from our local community for Sam Cataki over his visa application. He is a lovely guy and served in our Armed Forces for more than a decade.
"I have briefed the immigration minister and requested that the Home Office treats this as an urgent case. I have also asked for them to restore his right to work in the UK immediately.
"Having strong immigration rules is important but clearly the system should work better for our ex servicemen and women from Commonwealth countries."
A petition set up by the Neyland community in support of Mr Cataki has attracted nearly 20,000 signatures.

The 45-year-old man from the Swansea valley was riding a Kawasaki ZX 1000 when he crashed into the animal on the A4069 between Llangadog and Brynamman.
He was taken to Cardiff's University Hospital of Wales after the crash, which happened just before 18:00 BST on Sunday.
Dyfed-Powys Police is investigating.

Up to eight people are believed to have carried out the ram-raid at Martins newsagent in Prentice Place, Harlow, at about 01:45 BST.
The ATM was removed from an outside wall, causing "substantial damage", and was then loaded on to a van.
Essex Police are investigating whether the raid is linked to another at a Roydon newsagent earlier this month.
The cash machine contained a "five-figure sum", police said.
It is thought the thieves left in the van and a red car, believed to be an Audi. The digger, which was damaged in the raid, was left at the scene.
"We are especially keen to hear from anyone who saw the JCB, which is not the usual sort of vehicle to be seen on public roads late at night," Det Insp Jim Adams said.
"There was a previous incident involving the theft of a cash machine at a newsagent in Roydon on 16 September in which the machine and its contents were later recovered, and we are looking into whether these two incidents may be linked."
A Fijian father who served in the British Army for 12 years says he feels "neglected" after being refused UK citizenship.
[NEXT_CONCEPT]
A motorcyclist is in a critical condition in hospital after hitting a sheep in Carmarthenshire.
[NEXT_CONCEPT]
Thieves used a stolen JCB digger to smash into an Essex newsagent and steal a cash machine.