In short:

Members of HESTA will be unable to access most services until June, as the superannuation fund undertakes a planned outage to change its administration provider.

Advocacy group Super Consumers Australia says members are "rightly worried" about the lack of services, which follows recent market volatility.

HESTA has more than 1.05 million members and around $88 billion in funds under management, making it one of Australia's largest funds.