Definitely a grassbird, and the one here would be the montane Papuan, a split from Tawny which is a lowland species in NG, and vocally quite different. Did you hear any calls?
Taxonomic update:
Papuan Grassbird Megalurus macrurus
Common endemic resident
L23. Eight subspecies, nominate SE NG and Snow Mts. M. m. alpinus SE NG to Snow Mts larger and darker, more heavily streaked. M. m. montanus C NG alpine grasslands, darker. M. m. wahgiensis C NG mid-mountain grasslands, brighter more reddish above, whiter below. M. m. mayri N NG in lowland grasslands. M. m. harterti Huon Pen. darker. M. m. stresemanni Arfak Mts more ochraceous. M. t. interscapularis, Bismarcks paler with greyer underparts than New Guinea subspecies. Inhabits mostly mid-montane to subalpine grasslands, forest edge and shrublands, with two taxa currently ascribed to this species from lowland grasslands. Usually seen singly or in pairs, skulking but may call from atop tall kunai grass, often seen on ground, flies low with long ragged tail trailing. ID Similar to Tawny Grassbird but larger and darker and longer tailed, cap less rufous, vocally quite different.
Voc Much less varied than Tawny, harsh raspy “peechup” or “chewit” and scolding “chup” notes. Calls of Bismarck taxon seemingly undescribed. Similar Species Tawny Grassbird smaller, shorter tailed, has more rufous cap, paler underparts, different voice. Golden-headed Cisticola much smaller, shorter tail and distinct calls, from Australian Reed-Warbler or Gray’s Grasshopper Warbler by streaked upperparts and long ragged tail. TN Formerly lumped with Tawny Grassbird and some taxa ascribed to lowlands may still be wrongly allocated, requires further genetic and vocal studies.
