Re: 3M 4200 - Aussie Equivalent
Sika 291i has nearly identical specifications to 4200 (292i is very close to 5200). A fair number of the Sika products test better when used with their primers (3M recommends primers in fewer use cases). I think both of those are relatively available in Oz.
I’ve used Selleys MarineFlex (and a few of their other similar products) above the waterline but not below, not sure where they fall compared to 4200/5200.