A New Zealand company says the Papua New Guinea government owes it half a million dollars - and it's not paying.

In 2013, Wellington company Evaluation Consult was contracted to work for the Papua New Guinea government on their sustainable development plan.

The work was completed though the Papua New Guinea government made an error in not knowing it needed permission to have the quantity and cost of the contract was carried out. 

In June 2015, Wellington company Evaluation Consult won its court case in Papua New Guinea to recover the debt.

Evaluation Consult's executive director Kate Averill said the company worked in good faith with the Papua New Guinea government but has received no reason for the non-payment.

"They have to, as a government, honour their debts as a trading partner."

She said they were assured of payment in November but it hasn't been forthcoming.

"We still haven't been paid. That's half a million dollars to a New Zealand company. The work's been used and we still haven't been paid."

The group said there's been no payment despite warnings from Foreign Affairs Minister Murray McCully and Papua New Guinea’s High Commissioner to New Zealand.

Ms Averill said it raises bigger questions as to why the Papua New Guinea government can't pay their debts.

"Do they not want to? It seems very unfair and illegal.

"There's bigger questions here. New Zealand does give aid to the Papua New Guinea government. New Zealand encourages trade with Papua New Guinea and if a country can't pay its debts and doesn't act legally, then what is New Zealand's relationship with Papua New Guinea?

"Do we go as far as starting to put sanctions in place against a government that doesn't follow the law?"

Ms Averill said they are concerned other companies may also be left in the lurch.

"We want the legal process to be effective and that the debts are honoured. We want systems to be ethical, legal and honourable."