1. Specific to NSW
There is no state difference. This is governed by a national legal framework.
2. Naturist family picture depicting underage persons - is it classified as restricted content under Australian law?
In most circumstances, while videos that show people having sex are technically legally 'prohibited' online (which means that the Australian Communications and Media Authority can request that they are removed), it is generally not against the law to look at these videos for personal use.
However, there are some important exceptions. For example, it is a serious crime to view child abuse material (nude or sexual photos or videos of someone under 18). There are also crimes that can apply if you send or show x to other people who are under 18, even if itâ€™s a private place like your house.
From your description, family oriented photos are unlikely to be classified as restricted content even though it has under-aged person (not abused, not engaging in sexual
3. Is the naturist content illegal/compliant-able in some other way?
Under the Broadcasting Services Act 1992 (the BSA), all R18+ content and certain commercial MA15+ content online is prohibited or potentially prohibited unless it is behind a valid restricted access system to help prevent under-age viewing. Again from your description it does not appear that such website has breach any law.
Find out more at http://www.acma.gov.au/theACMA/acma-reviewing-access-restrictions-to-certain-online-content-1