Context. Gamma-ray detected radio-loud narrow-line Seyfert 1 (gamma-NLS1) galaxies constitute an small but interesting sample of the gamma-ray loud AGN. The most radio-loud gamma-NLS1, PKS 2004-447, is located in the southern hemisphere and is monitored by the multiwavelength monitoring program TANAMI in the radio regime. Aims. Here, we aim for the first detailed study of the radio morphology and long-term radio spectral evolution of PKS 2004-447, which are essential to understand the diversity of the radio properties of gamma-NLS1s Methods. The TANAMI VLBI monitoring program uses the Australian Long Baseline Array (LBA) and telescopes in Antarctica, Chile, New Zealand and South Africa to monitor the jets of radio-loud active galaxies in the southern hemisphere. Lower resolution radio flux density measurements at multiple radio frequencies over four years of observation were obtained with the Australia Telescope Compact Array (ATCA). Results. The TANAMI VLBI image at 8.4GHz shows an extended one-sided jet with a dominant compact VLBI core. The brightness temperature of the VLBI core of PKS 2004-447 is consistent with equipartition, but is an order of magnitude below other gamma-NLS1s with the sample value varying over two orders of magnitude. We find a compact morphology (< 11 kpc) and a persistent steep radio spectrum with moderate flux-density variability. Conclusions. PKS 2004-447 appears to be a unique member of the gamma-NLS1 sample. It exhibits blazar-like features, such as a flat featureless X-ray spectrum and a core dominated, one-sided parsec-scale jet with indications for relativistic beaming. However, our data also reveal properties atypical for blazars, such as a radio spectrum and large scale size consistent with Compact-Steep-Spectrum (CSS) sources, which are usually associated with young radio sources. These characteristics are unique among all gamma-NLS1s and extremely rare among gamma-ray loud AGN.