Disclosure is quite complex but there are some general rules and...

Disclosure is quite complex but there are some general rules and guidance. But changes depending on circumstance.
If a reasonable person would expect it to have a material effect on price or value it should be disclosed.

However generally there's no need for disclosure of confidential approaches to enter into a takeover, bid etc etc, provided its incomplete, its still confidential, and is not already the subject of a current takeover bid. Confidentiality is a key.

From recollection the ASX decided David Jones didn't need to disclose it in the end but don't quote me. But likewise ASX says Trading Halts should be used more often when issues arise and they can't make an immediate statement. Eg Rumors, media reports speculating especially when they reveal a number of facts. Pretty sure TAP last year went into a halt after media speculation of an asset sale which was fairly specific before they made an ann. Now bearing in mind Nexus appears to be dealing in an asset sale too, not a takeover, so there may be slight differences with David Jones but similar to TAP.

So on confidentiality if the media revealed Nexus was offered $600 mill for Crux by company X (and its accurate) then I think ASX would expect something from Nexus. If the article didn't mention price or company or anything specific, Nexus could just say they are in negotiations (there current stance). If it was something that was incorrect, eg there was no offer of $600 mill then ASX would expect Nexus to fix the rumour so to speak to prevent a false market. Hope this is making sense.

Anyway IMO Nexus aren't yet obliged to report the status of their negotiations.

For riveting bedtime reading try this
http://www.asxgroup.com.au/media/PDFs/gn08_continuous_disclosure.pdf