Buyers of a big Takapuna apartment project replacing The Block homes are the latest off-the-plan purchasers to be asked to pay more for units.

Letters have gone to Sargeson Apartments' buyers from the developer Auburn Development, detailing trouble on the Anzac St site where the television series was filmed.

"Construction costs have escalated to an unprecedented level and development finance is virtually unavailable in New Zealand.

"Demand for Takapuna apartments remains very strong and we believe will continue to do so as other competing apartment projects are put on hold or abandoned," a letter from Auburn Developments said.

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"Auburn has received an overseas offer of finance for the development. This finance is significantly more expensive than what was anticipated and Auburn is currently assessing the impact of the additional costs on the feasibility of the project.

"Auburn has analysed the project and has concluded that in order to proceed, the apartment sale prices need to be increased by 8 per cent. Auburn will provide you with a first right to purchase the same apartment at a price increase of 8 per cent. Your existing deposit does not need to be increased," the document said.

But one buyer said he faced a 12.5 per cent increase. Instead of $465,000, he would was being asked for $535,000 and he was unhappy about that.