The scrum-half, who has been capped 81 times by his country, is now in Wales after a season with Leeds.
And he claims to still be the All Blacks' best number nine, despite the presence of Byron Kelleher and Piri Weepu.
Marshall said: "I know I am still playing well enough to play for the All Blacks.
"It will come down to whether or not they want to pick a player that isn't playing in New Zealand.
"Personally, I'd pick the best New Zealanders wherever they are - it's about winning the World Cup at the end of the day.
"The fact that the position is up for contention shows that neither is better than the other.
Marshall fought out a long and bitter struggle for supremacy with Kelleher, and there is little love is lost between the pair.
"It's tough to comment because I had a lot of competition with Byron.
"Because of that you always think 'I don't know whether you are as good as you told me you are' and, then again, I don't know Piri Weepu that well either.
"The half-back position isn't settled in New Zealand. They haven't found a player that would take them through to the World Cup."