An independent review of how effectively and efficiently the Guardians is performing its function is carried out every five years, as required by the New Zealand Superannuation and Retirement Income Act 2001. The 2019 review was carried out by global business advisory firm Willis Towers Watson. The review says the Guardians is a global-best practice organisation and effective in carrying out its mandate. Previous independent reviews were carried out in 2014, 2009 and 2004.