Use of "completely rubbish"

I notice that it is rubbish means it is bad, but can we say it is completely rubbish meaning it is completely bad in everyday English? Do native speakers of English say that, other than the obviously correct it is complete rubbish?

In this instance, it's the noun "rubbish" being used as an adjectival metaphor, so the construction would be adv. + adj.

However, I think the phrase in British English is "complete rubbish", no? But you are correct, it is an idiomatic expression for "bad", which is an adjectival, not a noun. In that instance, you'd be using "complete rubbish" as adj. + noun, as a metaphorical adjective.

"Completely rubbish" would probably be most often used to refer to actual piles of rubbish, wherein nothing of value can be found.

I am a native speaker who says both 'completely rubbish' and 'complete rubbish'. I suspect I use them interchangeable as in 'This is complete(ly) rubbish', but I think kiamlaluno hints at the distinction.

He was a completely rubbish manager

This book is complete rubbish.

The first example has rubbish as an adjective and completely as an adverb, the second has rubbish as a noun and complete as an adjective. However,

He was completely rubbish as a manager.

He was a completely rubbish manager.

He was complete rubbish as a manager.

all sound okay to me. 'This book is completely rubbish' sounds odder, but, like I said, I suspect I say it. 'complete rubbish' really doesn't seem to be used in the US, as sometimes people simply don't understand me when I say it.

Rubbish is also a verb meaning "criticize severely and reject as worthless," and an adjective meaning "very bad; worthless or useless"; in both the cases, an adverb can be used with rubbish.

He has pointedly rubbished professional estimates of the development and running costs.
People might say I was a completely rubbish manager.