This study aimed to develop the ‘Healthy Eating Index for Older People’ (the index), based on New Zealand dietary guidelines, and measures the validity and reproducibility of a food frequency questionnaire (FFQ) to derive the index scores in older adults. In Auckland, New Zealand, participants (community-dwelling adults aged 65–74 years, [n = 273, 36% male]) completed a 109-item FFQ administered one month apart (FFQ1, FFQ2), with a four-day food record (4-DFR) collected in between. Adherence to the guidelines was scored using the index, comprising a total score (maximum = 100) and two sub-scores: adequacy (maximum = 60) and moderation (maximum = 40). A comparison of FFQ1 and FFQ2 determined reproducibility, and FFQ1 and 4-DFR determined validity. Higher index scores (from FFQ1) were associated with higher intakes of protein, fibre, vitamins and minerals and lower intakes of alcohol and saturated fats (nutrients from 4-DFR) after adjusting for age and sex (all p < 0.001). Total index and sub-indices correlation coefficients ranged from 0.42 to 0.77 (all p < 0.001); weighted kappa values ranged from 0.35 to 0.67; and mean differences were all <10% of FFQ1 for reproducibility and validity. The Bland–Altman analysis showed no bias for the total index score for reproducibility and validity. However, with validation, the sub-index scores became less reliable as food intake increased (p < 0.05). The index applied to the FFQ demonstrated good construct validity and reproducibility. Relative and absolute validity were acceptable, though caution is required when using the absolute sub-index scores. The index is suitable for measuring total diet quality in older New Zealand adults.