Treasury warned the Finance Minister - for caution over National's campaign cancer drugs promise

Just-released Budget documents reveal advice the 116-million-dollars saved by bringing back prescription co-payments was nowhere near enough to fund the treatments.

Officials also found the policy would undermine the independent model of drug-buying agency Pharmac.

The cash for cancer drugs was noticeably absent on Budget day - sparking widespread backlash from patients.

The coalition later committed more than 600-million-dollars of future spending to boost Pharmac's budget and fund up to 26 cancer drugs.